diff --git a/composer.json b/composer.json
index 7e46304..2cd9539 100644
--- a/composer.json
+++ b/composer.json
@@ -1,6 +1,6 @@
 {
-    "name":              "jrosset/arrayclasses",
-    "description":       "Classes for array embedding",
+    "name":              "jrosset/collections",
+    "description":       "Classes for collections",
     "keywords":          [ ],
 
     "minimum-stability": "stable",
@@ -15,7 +15,7 @@
     },
 
     "readme":            "README.md",
-    "homepage":          "https://git.jrosset.ovh/jrosset/PhpArrayClass",
+    "homepage":          "https://git.jrosset.ovh/jrosset/PhpCollections",
     "license":           "CC-BY-4.0",
     "authors":           [
         {
@@ -25,9 +25,9 @@
     ],
     "support":           {
         "email":  "jul.rosset@gmail.com",
-        "issues": "https://git.jrosset.ovh/jrosset/PhpArrayClass/issues",
-        "wiki":   "https://git.jrosset.ovh/jrosset/PhpArrayClass/wiki",
-        "docs":   "https://git.jrosset.ovh/jrosset/PhpArrayClass/wiki",
-        "source": "https://git.jrosset.ovh/jrosset/PhpArrayClass"
+        "issues": "https://git.jrosset.ovh/jrosset/PhpCollections/issues",
+        "wiki":   "https://git.jrosset.ovh/jrosset/PhpCollections/wiki",
+        "docs":   "https://git.jrosset.ovh/jrosset/PhpCollections/wiki",
+        "source": "https://git.jrosset.ovh/jrosset/PhpCollections"
     }
 }
\ No newline at end of file
diff --git a/src/ArrayClasses/ArrayClass.php b/src/ArrayClasses/ArrayClass.php
deleted file mode 100644
index 88b3ef5..0000000
--- a/src/ArrayClasses/ArrayClass.php
+++ /dev/null
@@ -1,17 +0,0 @@
-<?php
-
-namespace jrosset\ArrayClasses;
-
-/**
- * An array
- */
-class ArrayClass extends ImmutableArrayClass implements IArrayClass {
-    use TInternalArray;
-
-    /**
-     * @inheritDoc
-     */
-    public function toImmutable (): IImmutableArrayClass {
-        return new ImmutableArrayClass($this->array, $this->throwsForNonExistentElement());
-    }
-}
\ No newline at end of file
diff --git a/src/ArrayClasses/IArrayClass.php b/src/ArrayClasses/IArrayClass.php
deleted file mode 100644
index d38b7a4..0000000
--- a/src/ArrayClasses/IArrayClass.php
+++ /dev/null
@@ -1,44 +0,0 @@
-<?php
-
-namespace jrosset\ArrayClasses;
-
-/**
- * Interface for array classes
- *
- * @see IImmutableArrayClass Immutable version
- */
-interface IArrayClass extends IImmutableArrayClass {
-    /**
-     * Create or replace a cell
-     *
-     * @param mixed $cellName  The cell name/offset
-     * @param mixed $cellValue The cell new value
-     *
-     * @return $this
-     */
-    public function set ($cellName, $cellValue): self;
-    /**
-     * Add a cell (at the end)
-     *
-     * @param mixed $cellValue The cell new value
-     *
-     * @return $this
-     */
-    public function push ($cellValue): self;
-    /**
-     * Remove a cell
-     *
-     * @param mixed $cellName                    The cell name/offset
-     * @param bool  $throwsForNonExistentElement Throws an exception for nonexistent elements ?
-     *
-     * @return $this
-     */
-    public function del ($cellName, ?bool $throwsForNonExistentElement = null): self;
-
-    /**
-     * Create an immutable copy
-     *
-     * @return IImmutableArrayClass The immutable copy
-     */
-    public function toImmutable (): IImmutableArrayClass;
-}
\ No newline at end of file
diff --git a/src/ArrayClasses/IImmutableArrayClass.php b/src/ArrayClasses/IImmutableArrayClass.php
deleted file mode 100644
index 9908cef..0000000
--- a/src/ArrayClasses/IImmutableArrayClass.php
+++ /dev/null
@@ -1,41 +0,0 @@
-<?php
-
-namespace jrosset\ArrayClasses;
-
-use ArrayAccess;
-use Countable;
-use IteratorAggregate;
-use JsonSerializable;
-use Serializable;
-
-/**
- * Interface for immutable array classes
- *
- * @see IArrayClass Mutable version
- */
-interface IImmutableArrayClass extends IteratorAggregate, JsonSerializable, Serializable, Countable, ArrayAccess, IArrayCast {
-    /**
-     * Check if a cell exists
-     *
-     * @param mixed $cellName The cell name/offset
-     *
-     * @return bool Is the cell exists ?
-     */
-    public function has ($cellName): bool;
-    /**
-     * Get the value of a cell
-     *
-     * @param mixed $cellName                    The cell name/offset
-     * @param bool  $throwsForNonExistentElement Throws an exception for nonexistent elements ?
-     *
-     * @return mixed The cell value
-     */
-    public function get ($cellName, ?bool $throwsForNonExistentElement = null);
-
-    /**
-     * Create a mutable copy
-     *
-     * @return IArrayClass The mutable copy
-     */
-    public function toMutable (): IArrayClass;
-}
\ No newline at end of file
diff --git a/src/ArrayClasses/ImmutableArrayClass.php b/src/ArrayClasses/ImmutableArrayClass.php
deleted file mode 100644
index 31b7039..0000000
--- a/src/ArrayClasses/ImmutableArrayClass.php
+++ /dev/null
@@ -1,17 +0,0 @@
-<?php
-
-namespace jrosset\ArrayClasses;
-
-/**
- * An immutable array
- */
-class ImmutableArrayClass implements IImmutableArrayClass {
-    use TImmutableInternalArray;
-
-    /**
-     * @inheritDoc
-     */
-    public function toMutable (): IArrayClass {
-        return new ArrayClass($this->array, $this->throwsForNonExistentElement());
-    }
-}
\ No newline at end of file
diff --git a/src/ArrayClasses/ImmutableException.php b/src/ArrayClasses/ImmutableException.php
deleted file mode 100644
index 8f7dfb9..0000000
--- a/src/ArrayClasses/ImmutableException.php
+++ /dev/null
@@ -1,11 +0,0 @@
-<?php
-
-namespace jrosset\ArrayClasses;
-
-use LogicException;
-
-/**
- * Exception thrown when trying to modify an immutable element
- */
-class ImmutableException extends LogicException {
-}
\ No newline at end of file
diff --git a/src/ArrayClasses/ImmutableInsensitiveCaseArrayClass.php b/src/ArrayClasses/ImmutableInsensitiveCaseArrayClass.php
deleted file mode 100644
index ffd3269..0000000
--- a/src/ArrayClasses/ImmutableInsensitiveCaseArrayClass.php
+++ /dev/null
@@ -1,47 +0,0 @@
-<?php
-
-namespace jrosset\ArrayClasses;
-
-/**
- * An immutable array with insensitive case cell name
- */
-class ImmutableInsensitiveCaseArrayClass implements IImmutableArrayClass {
-    use TImmutableInternalArray {
-        __construct as private __construct__TImmutableInternalArray;
-        has as private has__TImmutableInternalArray;
-        get as private get__TImmutableInternalArray;
-    }
-
-    /**
-     * @inheritDoc
-     */
-    public function __construct ($initial = null, bool $throwsForNonExistentElement = true) {
-        $this->__construct__TImmutableInternalArray($initial, $throwsForNonExistentElement);
-
-        $arrayNew = [];
-        foreach ($this->array as $key => $value) {
-            $arrayNew[mb_strtolower($key)] = $value;
-        }
-        $this->array = $arrayNew;
-    }
-
-    /**
-     * @inheritDoc
-     */
-    public function has ($cellName): bool {
-        return $this->has__TImmutableInternalArray(mb_strtolower($cellName));
-    }
-    /**
-     * @inheritDoc
-     */
-    public function get ($cellName, ?bool $throwsForNonExistentElement = null) {
-        return $this->get__TImmutableInternalArray(mb_strtolower($cellName), $throwsForNonExistentElement);
-    }
-
-    /**
-     * @inheritDoc
-     */
-    public function toMutable (): IArrayClass {
-        return new InsensitiveCaseArrayClass($this->array, $this->throwsForNonExistentElement());
-    }
-}
\ No newline at end of file
diff --git a/src/ArrayClasses/InsensitiveCaseArrayClass.php b/src/ArrayClasses/InsensitiveCaseArrayClass.php
deleted file mode 100644
index 2978ad5..0000000
--- a/src/ArrayClasses/InsensitiveCaseArrayClass.php
+++ /dev/null
@@ -1,33 +0,0 @@
-<?php
-
-namespace jrosset\ArrayClasses;
-
-/**
- * An array with insensitive case cell name
- */
-class InsensitiveCaseArrayClass extends ImmutableInsensitiveCaseArrayClass implements IArrayClass {
-    use TInternalArray {
-        set as private set__TInternalArray;
-        del as private del__TInternalArray;
-    }
-
-    /**
-     * @inheritDoc
-     */
-    public function set ($cellName, $cellValue): self {
-        return $this->set__TInternalArray(mb_strtolower($cellName), $cellValue);
-    }
-    /**
-     * @inheritDoc
-     */
-    public function del ($cellName, ?bool $throwsForNonExistentElement = null): self {
-        return $this->del__TInternalArray(mb_strtolower($cellName), $throwsForNonExistentElement);
-    }
-
-    /**
-     * @inheritDoc
-     */
-    public function toImmutable (): IImmutableArrayClass {
-        return new ImmutableInsensitiveCaseArrayClass($this->array, $this->throwsForNonExistentElement());
-    }
-}
\ No newline at end of file
diff --git a/src/ArrayClasses/TImmutableInternalArray.php b/src/ArrayClasses/TImmutableInternalArray.php
deleted file mode 100644
index d8fbce8..0000000
--- a/src/ArrayClasses/TImmutableInternalArray.php
+++ /dev/null
@@ -1,159 +0,0 @@
-<?php
-
-namespace jrosset\ArrayClasses;
-
-use ArrayIterator;
-use InvalidArgumentException;
-use OutOfRangeException;
-use Traversable;
-
-trait TImmutableInternalArray {
-    /**
-     * @var array The internal array
-     */
-    protected array $array = [];
-    /**
-     * @var bool Throws an exception for nonexistent elements ?
-     */
-    private bool $throwsForNonExistentElement = true;
-
-    /**
-     * Initialize the array
-     *
-     * @param IArrayCast|array|null $initial                     The initial value
-     * @param bool                  $throwsForNonExistentElement Throws an exception for nonexistent elements ?
-     */
-    public function __construct ($initial = null, bool $throwsForNonExistentElement = true) {
-        $this->setThrowsForNonExistentElement($throwsForNonExistentElement);
-
-        if (is_array($initial)) {
-            $this->array = $initial;
-        }
-        elseif (is_null($initial)) {
-            $this->array = [];
-        }
-        elseif ($initial instanceof IArrayCast) {
-            $this->array = $initial->toArray();
-        }
-        else {
-            throw new InvalidArgumentException('The initial value is not valid: null, array or ' . IArrayCast::class);
-        }
-    }
-    /**
-     * Set information to dump
-     *
-     * @return array The information to dump
-     *
-     * @see https://www.php.net/manual/function.var-dump.php
-     */
-    public function __debugInfo (): array {
-        return [
-            'array' => $this->array,
-        ];
-    }
-
-    /**
-     * @inheritDoc
-     */
-    public function has ($cellName): bool {
-        return array_key_exists($cellName, $this->array);
-    }
-    /**
-     * @inheritDoc
-     */
-    public function get ($cellName, ?bool $throwsForNonExistentElement = null) {
-        if ($this->has($cellName)) {
-            return $this->array[$cellName];
-        }
-        if ($throwsForNonExistentElement ?? $this->throwsForNonExistentElement()) {
-            throw new OutOfRangeException();
-        }
-        return null;
-    }
-
-    /**
-     * @inheritDoc
-     */
-    public function toArray (): array {
-        return $this->array;
-    }
-
-    /**
-     * @inheritDoc
-     */
-    public function count (): int {
-        return count($this->array);
-    }
-
-    /**
-     * @inheritDoc
-     */
-    public function getIterator (): Traversable {
-        return new ArrayIterator($this->array);
-    }
-
-    /**
-     * @inheritDoc
-     */
-    public function offsetExists ($offset): bool {
-        return $this->has($offset);
-    }
-    /**
-     * @inheritDoc
-     */
-    public function offsetGet ($offset) {
-        return $this->get($offset);
-    }
-    /**
-     * @inheritDoc
-     */
-    public function offsetSet ($offset, $value): void {
-        throw new ImmutableException();
-    }
-    /**
-     * @inheritDoc
-     */
-    public function offsetUnset ($offset): void {
-        throw new ImmutableException();
-    }
-
-    /**
-     * @inheritDoc
-     */
-    public function serialize (): string {
-        return serialize($this->array);
-    }
-    /**
-     * @inheritDoc
-     */
-    public function unserialize ($data): void {
-        $this->array = unserialize($data);
-    }
-
-    /**
-     * @inheritDoc
-     */
-    public function jsonSerialize () {
-        return $this->array;
-    }
-
-    /**
-     * Throws an exception for nonexistent elements ?
-     *
-     * @return bool Throws an exception for nonexistent elements ?
-     */
-    public function throwsForNonExistentElement (): bool {
-        return $this->throwsForNonExistentElement;
-    }
-    /**
-     * Set if throws an exception for nonexistent elements
-     *
-     * @param bool $throwsForNonExistentElement Throws an exception for nonexistent elements ?
-     *
-     * @return $this
-     */
-    public function setThrowsForNonExistentElement (bool $throwsForNonExistentElement): self {
-        $this->throwsForNonExistentElement = $throwsForNonExistentElement;
-        return $this;
-    }
-}
\ No newline at end of file
diff --git a/src/ArrayClasses/TInternalArray.php b/src/ArrayClasses/TInternalArray.php
deleted file mode 100644
index f136d31..0000000
--- a/src/ArrayClasses/TInternalArray.php
+++ /dev/null
@@ -1,45 +0,0 @@
-<?php
-
-namespace jrosset\ArrayClasses;
-
-use OutOfRangeException;
-
-trait TInternalArray {
-    /**
-     * @inheritDoc
-     */
-    public function set ($cellName, $cellValue): self {
-        $this->array[$cellName] = $cellValue;
-        return $this;
-    }
-    /**
-     * @inheritDoc
-     */
-    public function push ($cellValue): self {
-        return $this->set($this->count(), $cellValue);
-    }
-    /**
-     * @inheritDoc
-     */
-    public function del ($cellName, ?bool $throwsForNonExistentElement = null): self {
-        if ($throwsForNonExistentElement ?? $this->throwsForNonExistentElement() && !$this->has($cellName)) {
-            throw new OutOfRangeException();
-        }
-        unset($this->array[$cellName]);
-
-        return $this;
-    }
-
-    /**
-     * @inheritDoc
-     */
-    public function offsetSet ($offset, $value): void {
-        $this->set($offset, $value);
-    }
-    /**
-     * @inheritDoc
-     */
-    public function offsetUnset ($offset): void {
-        $this->del($offset);
-    }
-}
\ No newline at end of file
diff --git a/src/Collections/Collection.php b/src/Collections/Collection.php
new file mode 100644
index 0000000..fba5195
--- /dev/null
+++ b/src/Collections/Collection.php
@@ -0,0 +1,53 @@
+<?php
+
+namespace jrosset\Collections;
+
+/**
+ * A collection
+ *
+ * @psalm-template     TKey of array-key
+ * @template-covariant TValue
+ * @template-extends ImmutableCollection<TKey, TValue>
+ * @template-implements ICollection<TKey, TValue>
+ */
+class Collection extends ImmutableCollection implements ICollection {
+    /**
+     * @inheritDoc
+     */
+    public function set ($key, $value): self {
+        return $this->_set($key, $value);
+    }
+    /**
+     * @inheritDoc
+     */
+    public function add ($value): self {
+        $this->elements[] = $value;
+        return $this;
+    }
+
+    /**
+     * @inheritDoc
+     */
+    public function clear (): self {
+        $this->_initialize();
+        return $this;
+    }
+    /**
+     * @inheritDoc
+     */
+    public function remove ($key): self {
+        unset($this->elements[$this->_normalizeKey($key)]);
+        return $this;
+    }
+    /**
+     * @inheritDoc
+     */
+    public function removeValue ($value, bool $strict = false): self {
+        foreach ($this->elements as $currentKey => $currentValue) {
+            if (($strict && $value === $currentValue) || (!$strict && $value == $currentValue)) {
+                unset($this->elements[$currentKey]);
+            }
+        }
+        return $this;
+    }
+}
\ No newline at end of file
diff --git a/src/ArrayClasses/IArrayCast.php b/src/Collections/IArrayCast.php
similarity index 59%
rename from src/ArrayClasses/IArrayCast.php
rename to src/Collections/IArrayCast.php
index 6057ed4..2226353 100644
--- a/src/ArrayClasses/IArrayCast.php
+++ b/src/Collections/IArrayCast.php
@@ -1,15 +1,19 @@
 <?php
 
-namespace jrosset\ArrayClasses;
+namespace jrosset\Collections;
 
 /**
  * Interface for PHP-native array cast
+ *
+ * @psalm-template     TKey of array-key
+ * @template-covariant TValue
  */
 interface IArrayCast {
     /**
      * Transform to a PHP-native array
      *
      * @return array The native array
+     * @psalm-return array<TKey, TValue>
      */
     public function toArray (): array;
 }
\ No newline at end of file
diff --git a/src/Collections/ICollection.php b/src/Collections/ICollection.php
new file mode 100644
index 0000000..b0dff6d
--- /dev/null
+++ b/src/Collections/ICollection.php
@@ -0,0 +1,63 @@
+<?php
+
+namespace jrosset\Collections;
+
+/**
+ * Interface for a collection
+ *
+ * @psalm-template     TKey of array-key
+ * @template-covariant TValue
+ * @template-extends IImmutableCollection<TKey, TValue>
+ */
+interface ICollection extends IImmutableCollection {
+    /**
+     * Set a value to a key
+     *
+     * @param array-key    $key   The key
+     * @param mixed        $value The value
+     *
+     * @psalm-param TKey   $key
+     * @psalm-param TValue $value
+     *
+     * @return $this
+     */
+    public function set ($key, $value): self;
+    /**
+     * Add a value
+     *
+     * @param mixed        $value The value
+     *
+     * @psalm-param TValue $value
+     *
+     * @return $this
+     */
+    public function add ($value): self;
+
+    /**
+     * Empties the collection
+     *
+     * @return $this
+     */
+    public function clear (): self;
+    /**
+     * Delete a key
+     *
+     * @param array-key  $key The key
+     *
+     * @psalm-param TKey $key
+     *
+     * @return $this
+     */
+    public function remove ($key): self;
+    /**
+     * Delete all instances of a value
+     *
+     * @param mixed        $value  The value
+     * @param bool         $strict Strict comparison ?
+     *
+     * @psalm-param TValue $value
+     *
+     * @return $this
+     */
+    public function removeValue ($value, bool $strict = false): self;
+}
\ No newline at end of file
diff --git a/src/Collections/IImmutableCollection.php b/src/Collections/IImmutableCollection.php
new file mode 100644
index 0000000..131963c
--- /dev/null
+++ b/src/Collections/IImmutableCollection.php
@@ -0,0 +1,127 @@
+<?php
+
+namespace jrosset\Collections;
+
+use ArrayAccess;
+use Closure;
+use Countable;
+use IteratorAggregate;
+use JsonSerializable;
+use Serializable;
+
+/**
+ * Interface for an immutable (read-only) collection
+ *
+ * @psalm-template     TKey of array-key
+ * @template-covariant TValue
+ * @template-extends IteratorAggregate<TKey, TValue>
+ * @template-extends ArrayAccess<TKey, TValue>
+ * @template-extends IArrayCast<TKey, TValue>
+ */
+interface IImmutableCollection extends IteratorAggregate, JsonSerializable, Serializable, Countable, ArrayAccess, IArrayCast {
+    /**
+     * Checks if the collection is empty
+     *
+     * @return bool TRUE if the collection is empty, FALSE otherwise
+     */
+    public function empty (): bool;
+
+    /**
+     * Checks if a key exists
+     *
+     * @param array-key  $key The key
+     *
+     * @psalm-param TKey $key
+     *
+     * @return bool TRUE if the key exists, FALSE otherwise
+     */
+    public function exists ($key): bool;
+    /**
+     * Checks if contains a value
+     *
+     * @param mixed        $value  The value
+     * @param bool         $strict Strict comparison ?
+     *
+     * @psalm-param TValue $value
+     *
+     * @return bool TRUE if the value exists, FALSE otherwise
+     */
+    public function contains ($value, bool $strict = false): bool;
+
+    /**
+     * Get the value of a key or null if not found
+     *
+     * @param array-key  $key The key
+     *
+     * @psalm-param TKey $key
+     *
+     * @return mixed The value
+     * @psalm-return TValue|null
+     */
+    public function get ($key);
+    /**
+     * Get the first key of a value or null if not found
+     *
+     * @param mixed        $value  The value
+     * @param bool         $strict Strict comparison ?
+     *
+     * @psalm-param TValue $value
+     *
+     * @return array-key
+     * @psalm-return TKey|null
+     */
+    public function key ($value, bool $strict = false);
+
+    /**
+     * Extract a slice of the collection
+     *
+     * Preserves the keys
+     *
+     * @param int      $offset The start offset
+     * @param int|null $length The maximum length. Null if until end of the collection
+     *
+     * @return static
+     * @psalm-return static<TKey, TValue>
+     */
+    public function slice (int $offset, ?int $length = null): IImmutableCollection;
+
+    /**
+     * Get a collection of all elements that satisfy predicate $filter
+     *
+     * @param Closure                          $filter The filtering predicate
+     *
+     * @psalm-param Closure(TKey, TValue):bool $filter
+     *
+     * @return static The result collection
+     * @psalm-return static<TKey, TValue>
+     */
+    public function filter (Closure $filter): IImmutableCollection;
+    /**
+     * A new collection with $process applied on all elements
+     *
+     * @psalm-template TResultValue
+     *
+     * @param Closure                                   $process The process function to apply on each element
+     *
+     * @psalm-param Closure(TKey, TValue): TResultValue $process
+     *
+     * @return static
+     * @psalm-return static<TKey, TResultValue>
+     */
+    public function map (Closure $process): IImmutableCollection;
+
+    /**
+     * The list of all keys
+     *
+     * @return static The list of all keys
+     * @psalm-return static<int, TKey>
+     */
+    public function keys (): IImmutableCollection;
+    /**
+     * The list of all values
+     *
+     * @return static The list of all values
+     * @psalm-return static<int, TValue>
+     */
+    public function values (): IImmutableCollection;
+}
\ No newline at end of file
diff --git a/src/Collections/ImmutableCollection.php b/src/Collections/ImmutableCollection.php
new file mode 100644
index 0000000..b58b1da
--- /dev/null
+++ b/src/Collections/ImmutableCollection.php
@@ -0,0 +1,271 @@
+<?php
+
+namespace jrosset\Collections;
+
+use ArrayIterator;
+use Closure;
+use InvalidArgumentException;
+use JsonException;
+use Traversable;
+
+/**
+ * An immutable collection
+ *
+ * @psalm-template     TKey of array-key
+ * @template-covariant TValue
+ * @template-implements IImmutableCollection<TKey, TValue>
+ */
+class ImmutableCollection implements IImmutableCollection {
+    /**
+     * @var array The internal array of elements
+     * @psalm-var array<TKey, TValue>
+     *
+     * @internal
+     */
+    protected array $elements;
+
+    /**
+     * Initialise a new collection
+     *
+     * @param array|Traversable|null $other The initial values
+     *
+     * @psalm-param array<TKey, TValue>|Traversable<TKey, TValue>|null
+     *
+     * @throws InvalidArgumentException If the initial values aren't valid
+     */
+    public function __construct ($other = null) {
+        $this->_initialize($other);
+    }
+
+    /**
+     * Initialise the internals elements
+     *
+     * @param array|Traversable|null $other The initial values
+     *
+     * @psalm-param array<TKey, TValue>|Traversable<TKey, TValue>|null
+     *
+     * @throws InvalidArgumentException If the initial values aren't valid
+     * @internal
+     *
+     */
+    protected function _initialize ($other = null) {
+        $this->elements = [];
+        if ($other !== null) {
+            if (!is_array($other) && !$other instanceof Traversable) {
+                throw new InvalidArgumentException('The initial values must be an array or a ' . IArrayCast::class . ' instance');
+            }
+
+            foreach ($other as $key => $value) {
+                $this->_set($key, $value);
+            }
+        }
+    }
+    /**
+     * Internally set a value to a key
+     *
+     * @param array-key    $key   The key
+     * @param mixed        $value The value
+     *
+     * @psalm-param TKey   $key
+     * @psalm-param TValue $value
+     *
+     * @return $this
+     * @internal
+     *
+     */
+    protected function _set ($key, $value): self {
+        $this->elements[$this->_normalizeKey($key)] = $value;
+        return $this;
+    }
+    /**
+     * Internally add a value
+     *
+     * @param mixed        $value The value
+     *
+     * @psalm-param TValue $value
+     *
+     * @return $this
+     * @internal
+     *
+     */
+    protected function _add ($value): self {
+        $this->elements[] = $value;
+        return $this;
+    }
+    /**
+     * Normalize a key
+     *
+     * @param array-key  $key The key to normalize
+     *
+     * @psalm-param TKey $key
+     *
+     * @return array-key The normalized key
+     * @psalm-return TKey
+     */
+    protected function _normalizeKey ($key) {
+        return $key;
+    }
+
+    /**
+     * @inheritDoc
+     */
+    public function getIterator () {
+        return new ArrayIterator($this->elements);
+    }
+
+    /**
+     * @inheritDoc
+     */
+    public function offsetExists ($offset): bool {
+        return $this->exists($offset);
+    }
+    /**
+     * @inheritDoc
+     */
+    public function offsetGet ($offset) {
+        return $this->get($offset);
+    }
+    /**
+     * @inheritDoc
+     */
+    public function offsetSet ($offset, $value) {
+        throw new ImmutableException();
+    }
+    /**
+     * @inheritDoc
+     */
+    public function offsetUnset ($offset) {
+        throw new ImmutableException();
+    }
+
+    /**
+     * @inheritDoc
+     */
+    public function serialize (): ?string {
+        return serialize($this->elements);
+    }
+    /**
+     * @inheritDoc
+     */
+    public function unserialize ($data) {
+        $this->_initialize(unserialize($data));
+    }
+
+    /**
+     * @inheritDoc
+     */
+    public function toArray (): array {
+        return $this->elements;
+    }
+
+    /**
+     * @inheritDoc
+     */
+    public function count (): int {
+        return count($this->elements);
+    }
+    /**
+     * @inheritDoc
+     */
+    public function empty (): bool {
+        return $this->count() === 0;
+    }
+
+    /**
+     * @inheritDoc
+     */
+    public function exists ($key): bool {
+        return isset($this->elements[$this->_normalizeKey($key)]);
+    }
+    /**
+     * @inheritDoc
+     */
+    public function contains ($value, bool $strict = false): bool {
+        return in_array($value, $this->elements, $strict);
+    }
+
+    /**
+     * @inheritDoc
+     */
+    public function get ($key) {
+        return $this->elements[$this->_normalizeKey($key)] ?? null;
+    }
+    /**
+     * @inheritDoc
+     */
+    public function key ($value, bool $strict = false) {
+        foreach ($this->elements as $currentKey => $currentValue) {
+            if (($strict && $value === $currentValue) || (!$strict && $value == $currentValue)) {
+                return $currentKey;
+            }
+        }
+        return null;
+    }
+
+    /**
+     * @inheritDoc
+     */
+    public function slice (int $offset, ?int $length = null): IImmutableCollection {
+        $output = new static();
+
+        $currentIndex = 0;
+        foreach ($this->elements as $key => $value) {
+            if ($currentIndex++ < $offset) {
+                continue;
+            }
+
+            $output->_set($key, $value);
+            if ($output->count() >= $length) {
+                break;
+            }
+        }
+
+        return $output;
+    }
+
+    /**
+     * @inheritDoc
+     */
+    public function filter (Closure $filter): IImmutableCollection {
+        $output = new static();
+        foreach ($this->elements as $key => $value) {
+            if (!$filter($key, $value)) {
+                continue;
+            }
+            $output->_set($key, $value);
+        }
+        return $output;
+    }
+    /**
+     * @inheritDoc
+     */
+    public function map (Closure $process): IImmutableCollection {
+        $output = new static();
+        foreach ($this->elements as $key => $value) {
+            $output->_set($key, $process($key, $value));
+        }
+        return $output;
+    }
+
+    /**
+     * @inheritDoc
+     */
+    public function keys (): IImmutableCollection {
+        return new static(array_keys($this->elements));
+    }
+    /**
+     * @inheritDoc
+     */
+    public function values (): IImmutableCollection {
+        return new static(array_values($this->elements));
+    }
+
+    /**
+     * @inheritDoc
+     *
+     * @throws JsonException
+     */
+    public function jsonSerialize () {
+        return json_encode($this->elements, JSON_THROW_ON_ERROR);
+    }
+}
\ No newline at end of file
diff --git a/src/Collections/ImmutableException.php b/src/Collections/ImmutableException.php
new file mode 100644
index 0000000..a517bf7
--- /dev/null
+++ b/src/Collections/ImmutableException.php
@@ -0,0 +1,11 @@
+<?php
+
+namespace jrosset\Collections;
+
+use LogicException;
+
+/**
+ * Exception thrown when trying to modify an immutable collection
+ */
+class ImmutableException extends LogicException {
+}
\ No newline at end of file
diff --git a/src/Collections/InsensitiveCaseKeyCollection.php b/src/Collections/InsensitiveCaseKeyCollection.php
new file mode 100644
index 0000000..12b6321
--- /dev/null
+++ b/src/Collections/InsensitiveCaseKeyCollection.php
@@ -0,0 +1,14 @@
+<?php
+
+namespace jrosset\Collections;
+
+/**
+ * A collection with insensitive case keys
+ *
+ * @psalm-template     TKey of array-key
+ * @template-covariant TValue
+ * @template-extends Collection<TKey, TValue>
+ */
+class InsensitiveCaseKeyCollection extends Collection {
+    use TInsensitiveCaseKey;
+}
\ No newline at end of file
diff --git a/src/Collections/InsensitiveCaseKeyImmutableCollection.php b/src/Collections/InsensitiveCaseKeyImmutableCollection.php
new file mode 100644
index 0000000..166b202
--- /dev/null
+++ b/src/Collections/InsensitiveCaseKeyImmutableCollection.php
@@ -0,0 +1,14 @@
+<?php
+
+namespace jrosset\Collections;
+
+/**
+ * An immutable collection with insensitive case keys
+ *
+ * @psalm-template     TKey of array-key
+ * @template-covariant TValue
+ * @template-extends ImmutableCollection<TKey, TValue>
+ */
+class InsensitiveCaseKeyImmutableCollection extends ImmutableCollection implements IImmutableCollection {
+    use TInsensitiveCaseKey;
+}
\ No newline at end of file
diff --git a/src/Collections/TInsensitiveCaseKey.php b/src/Collections/TInsensitiveCaseKey.php
new file mode 100644
index 0000000..d2d4757
--- /dev/null
+++ b/src/Collections/TInsensitiveCaseKey.php
@@ -0,0 +1,19 @@
+<?php
+
+namespace jrosset\Collections;
+
+/**
+ * Implementation for insensitive case key
+ */
+trait TInsensitiveCaseKey {
+    /**
+     * Normalize a key
+     *
+     * @param array-key $key The key to normalize
+     *
+     * @return array-key The normalized key
+     */
+    protected function _normalizeKey ($key) {
+        return mb_strtolower($key);
+    }
+}
\ No newline at end of file
diff --git a/tests/test.php b/tests/test.php
new file mode 100644
index 0000000..6d404ab
--- /dev/null
+++ b/tests/test.php
@@ -0,0 +1,21 @@
+<?php
+
+use jrosset\Collections\InsensitiveCaseKeyCollection;
+use jrosset\Collections\InsensitiveCaseKeyImmutableCollection;
+
+require_once __DIR__ . '/../vendor/autoload.php';
+
+$readOnlyCollection = new InsensitiveCaseKeyImmutableCollection(
+    [
+        'Foo' => 'bar',
+        'foo' => 'foobar',
+    ]
+);
+
+echo 'Size : ' . $readOnlyCollection->count() . PHP_EOL;
+echo 'Foo = ' . $readOnlyCollection->get('Foo') . PHP_EOL;
+echo '-----' . PHP_EOL;
+
+$collection = new InsensitiveCaseKeyCollection($readOnlyCollection);
+$collection->add(28);
+echo 'Size : ' . $collection->count() . PHP_EOL;
\ No newline at end of file
diff --git a/tests/test_class.php b/tests/test_class.php
new file mode 100644
index 0000000..15fc6d3
--- /dev/null
+++ b/tests/test_class.php
@@ -0,0 +1,37 @@
+<?php
+/** @noinspection PhpIllegalPsrClassPathInspection */
+
+use jrosset\Collections\Collection;
+
+require_once __DIR__ . '/../vendor/autoload.php';
+
+class Measure {
+    private int    $value;
+    private string $unit;
+
+    public function __construct (int $value, string $unit = 's') {
+        $this->value = $value;
+        $this->unit = $unit;
+    }
+
+    public function asString (): string {
+        return $this->value . $this->unit;
+    }
+}
+
+/**
+ * @return Collection<int, Measure>
+ */
+function getMeasures (): Collection {
+    $measures = new Collection();
+    $measures->add(new Measure(38));
+    $measures->add(new Measure(27));
+    $measures->add(new Measure(34));
+
+    return $measures;
+}
+
+echo 'Measures :' . PHP_EOL;
+foreach (getMeasures() as $no => $measure) {
+    echo "\t#" . $no . ' = ' . $measure->asString() . PHP_EOL;
+}
\ No newline at end of file