commit 512d04ff19a93027bdac30a142b0dd91144075f6
Author: CismonX <admin@cismon.net>
Date:   Tue Mar 20 22:44:25 2018 +0800

    initial commit

diff --git a/LICENSE b/LICENSE
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+++ b/LICENSE
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+The MIT License (MIT)
+
+Copyright (c) 2018 CismonX
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in
+all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+THE SOFTWARE.
\ No newline at end of file
diff --git a/README.md b/README.md
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+# ext-collections
+
+## 1. Introduction
+
+This PHP extension provides a set of useful functional-style operations on PHP arrays, which makes array manipulation simple and scalable.
+
+Method names and functionalities are inspired by [Kotlin.Collections](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/).
+
+It may take me a really long time to fully implement this extension. Feel free to contribute.
+
+## 2. Documentation
+
+See [stubs](stubs/) directory for signature of all classes and methods of this extension, with PHPDoc.
+
+## 3. Example
+
+Here is a simple example.
+
+```php
+$employees = [
+    ['name' => 'Alice', 'sex' => 'female', 'age' => 35],
+    ['name' => 'Bob', 'sex' => 'male', 'age' => 29],
+    ['name' => 'David', 'sex' => 'male', 'age' => 40],
+    ['name' => 'Benjamin', 'sex' => 'male', 'age' => 32]
+];
+// Trying to get an array of names of male employees,
+// sorted by the descending order of their age.
+$names = Collection::init($employees)
+    ->filter(function ($it) {
+        return $it->get('sex') == 'male';
+    })
+    ->sortedByDescending(function ($it) {
+        return $it->get('age');
+    })
+    ->map(function ($it) {
+        return $it->get('name');
+    })
+    ->toArray();
+```
+
diff --git a/stubs/Collection.php b/stubs/Collection.php
new file mode 100644
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+++ b/stubs/Collection.php
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+<?php
+
+/**
+ * Wrapper for PHP array and provide functional-style operations.
+ */
+class Collection implements ArrayAccess, Countable
+{
+    /**
+     * Private constructor.
+     * The Collection class should be initialized with static method Collection::init($data).
+     */
+    private function __construct() {}
+
+    /**
+     * Adds all elements of the given elements collection to this Collection.
+     *
+     * @param array|Collection $elements
+     * @return bool
+     */
+    function addAll($elements) {}
+
+    /**
+     * Returns true if all elements match the given predicate.
+     *
+     * @param callable $predicate ($value, $key) -> bool
+     * @return bool
+     */
+    function all($predicate) {}
+
+    /**
+     * Returns true if at least one element matches the given predicate.
+     *
+     * @param callable $predicate ($value, $key) -> bool
+     * @return bool
+     */
+    function any($predicate) {}
+
+    /**
+     * Returns a collection containing key-value pairs provided by transform function applied
+     * to elements of the given collection.
+     *
+     * @param callable $transform ($value) -> Pair
+     * @return Collection
+     */
+    function associate($transform) {}
+
+    /**
+     * Populates and returns the destination collection with key-value pairs provided by transform
+     * function applied to each element of the given collection.
+     *
+     * @param Collection $destination
+     * @param callable $transform ($value) -> Pair
+     * @return Collection
+     */
+    function associateTo($destination, $transform) {}
+
+    /**
+     * Returns a collection containing the elements from the given collection indexed by the key
+     * returned from keySelector function applied to each element.
+     *
+     * @param callable $key_selector ($value) -> $key
+     * @return Collection
+     */
+    function associateBy($key_selector) {}
+
+    /**
+     * Populates and returns the destination collection with key-value pairs, where key is
+     * provided by the keySelector function applied to each element of the given collection
+     * and value is the element itself.
+     *
+     * @param Collection $destination
+     * @param callable $key_selector ($value) -> $key
+     * @return Collection
+     */
+    function associateByTo($destination, $key_selector) {}
+
+    /**
+     * Returns an average value of elements in the collection.
+     *
+     * @return double
+     */
+    function average() {}
+
+    /**
+     * Checks if all elements in the specified collection are contained in this collection.
+     *
+     * @param Collection $collection
+     * @return bool
+     */
+    function containsAll($collection) {}
+
+    /**
+     * Checks if the array contains the given key.
+     *
+     * @param int|string $element
+     * @return bool
+     */
+    function containsKey($element) {}
+
+    /**
+     * Check if the array maps one or more keys to the specified value.
+     *
+     * @param mixed $element
+     * @return bool
+     */
+    function containsValue($element) {}
+
+    /**
+     * Returns new array which is a copy of the original array.
+     *
+     * @param Collection $collection
+     * @param int $new_size[optional]
+     * @return Collection
+     */
+    function copyOf($collection, $new_size) {}
+
+    /**
+     * Returns new array which is a copy of range of original array.
+     *
+     * @param int $from_index
+     * @param int $to_index
+     * @return Collection
+     */
+    function copyOfRange($from_index, $to_index) {}
+
+    /**
+     * Returns the number of elements in this array.
+     *
+     * @return int
+     */
+    function count() {}
+
+    /**
+     * Returns a collection containing only distinct elements from the given array.
+     *
+     * @return Collection
+     */
+    function distinct() {}
+
+    /**
+     * Returns a collection containing only elements from the given array having distinct keys
+     * returned by the given selector function.
+     *
+     * @param callable $selector ($value, $key) -> $key
+     * @return Collection
+     */
+    function distinctBy($selector) {}
+
+    /**
+     * Returns a list containing all elements except first n elements.
+     *
+     * @param int $n
+     * @return Collection
+     */
+    function drop($n) {}
+
+    /**
+     * Returns a collection containing all elements except last n elements.
+     *
+     * @param int $n
+     * @return Collection
+     */
+    function dropLast($n) {}
+
+    /**
+     * Returns a collection containing all elements except last elements that satisfy
+     * the given predicate.
+     *
+     * @param callable $predicate ($value, $key) -> bool
+     * @return Collection
+     */
+    function dropLastWhile($predicate) {}
+
+    /**
+     * Returns a collection containing all elements except first elements that satisfy
+     * the given predicate.
+     *
+     * @param $predicate $predicate ($value, $key) -> bool
+     * @return Collection
+     */
+    function dropWhile($predicate) {}
+
+    /**
+     * Fills original array with the provided value.
+     *
+     * @param mixed $element
+     * @param int $from_index[optional]
+     * @param int $to_index[optional]
+     * @return Collection
+     */
+    function fill($element, $from_index, $to_index) {}
+
+    /**
+     * Returns a collection containing only elements matching the given predicate.
+     *
+     * @param callable $predicate ($value, $key) -> bool
+     * @return Collection
+     */
+    function filter($predicate) {}
+
+    /**
+     * Returns a collection containing only elements not matching the given predicate.
+     *
+     * @param callable $predicate ($value, $key) -> bool
+     * @return Collection
+     */
+    function filterNot($predicate) {}
+
+    /**
+     * Appends all elements not matching the given predicate to the given destination.
+     *
+     * @param Collection $destination
+     * @param callable $predicate ($value, $key) -> bool
+     * @return Collection
+     */
+    function filterNotTo($destination, $predicate) {}
+
+    /**
+     * Appends all elements matching the given predicate to the given destination.
+     *
+     * @param Collection $destination
+     * @param callable $predicate ($value, $key) -> bool
+     * @return Collection
+     */
+    function filterTo($destination, $predicate) {}
+
+    /**
+     * Returns the first element matching the given predicate, or null if no such
+     * element was found.
+     *
+     * @param callable $predicate ($value) -> bool
+     * @return mixed
+     */
+    function find($predicate) {}
+
+    /**
+     * Returns the last element matching the given predicate, or null if no such
+     * element was found.
+     *
+     * @param callable $predicate ($value) -> bool
+     * @return mixed
+     */
+    function findLast($predicate) {}
+
+    /**
+     * Returns the first element matching the given predicate.
+     *
+     * @param callable $predicate[optional] ($value, $key) -> bool
+     * @return mixed
+     */
+    function first($predicate) {}
+
+    /**
+     * Returns a collection of all elements yielded from results of transform function
+     * being invoked on each element of original collection.
+     *
+     * @param callable $transform ($value, $key) -> array|Collection
+     * @return Collection
+     */
+    function flatMap($transform) {}
+
+    /**
+     * Appends all elements yielded from results of transform function being invoked on each element of
+     * original collection, to the given destination.
+     *
+     * @param Collection $destination
+     * @param callable $transform ($value, $key) -> array|Collection
+     * @return Collection
+     */
+    function flatMapTo($destination, $transform) {}
+
+    /**
+     * Returns a single list of all elements from all collections in the given collection.
+     *
+     * @return Collection
+     */
+    function flatten() {}
+
+    /**
+     * Accumulates value starting with initial value and applying operation from left to
+     * right to current accumulator value and each element.
+     *
+     * @param mixed $initial
+     * @param callable $operation ($acc, $value, $key) -> $new_acc
+     * @return mixed
+     */
+    function fold($initial, $operation) {}
+
+    /**
+     * Accumulates value starting with initial value and applying operation from right to
+     * left to each element and current accumulator value.
+     *
+     * @param mixed $initial
+     * @param callable $operation ($acc, $value, $key) -> $new_acc
+     * @return mixed
+     */
+    function foldRight($initial, $operation) {}
+
+    /**
+     * Performs the given action on each element.
+     *
+     * @param callable $action ($value, $key) -> void
+     * @return void
+     */
+    function forEach($action) {}
+
+    /**
+     * Returns an element at the given key or the result of calling the `default` function
+     * if the index is out of bounds of this array.
+     *
+     * @param int|string $key
+     * @param callable $default[optional] ($key) -> $value
+     * @return Collection
+     */
+    function get($key, $default) {}
+
+    /**
+     * Groups values returned by the value_transform function applied to each element of the
+     * original array by the key returned by the given key_selector function applied to the element
+     * and returns a collection where each group key is associated with a list of corresponding values.
+     *
+     * @param callable $key_selector ($value, $key) -> $new_key
+     * @param callable $value_transform[optional] ($value) -> $new_value
+     * @return Collection
+     */
+    function groupBy($key_selector, $value_transform) {}
+
+    /**
+     * Groups values returned by the value_transform function applied to each element of the original
+     * collection by the key returned by the given key_selector function applied to the element and
+     * puts to the destination collection each group key associated with a list of corresponding values.
+     *
+     * @param Collection $destination
+     * @param callable $key_selector ($value, $key) -> $new_key
+     * @param callable $value_transform[optional] ($value) -> $new_value
+     * @return Collection
+     */
+    function groupByTo($destination, $key_selector, $value_transform) {}
+
+    /**
+     * Returns first key of element, or null if the array does not contain element.
+     *
+     * @param mixed $element
+     * @return int|string|null
+     */
+    function indexOf($element) {}
+
+    /**
+     * Returns key of the first element matching the given predicate, or null if the array does
+     * not contain such element.
+     *
+     * @param callable $predicate ($value) -> bool
+     * @return int|string|null
+     */
+    function indexOfFirst($predicate) {}
+
+    /**
+     * Returns key of the last element matching the given predicate, or null if the array does
+     * not contain such element.
+     *
+     * @param callable $predicate ($value) -> bool
+     * @return int|string|null
+     */
+    function indexOfLast($predicate) {}
+
+    /**
+     * Initialize collection with given data.
+     *
+     * @param array $data[$optional]
+     * @return Collection
+     */
+    static function init($data) {}
+
+    /**
+     * Returns a collection containing all elements that are contained by both this collection and the
+     * specified collection.
+     *
+     * @param array|Collection $other
+     */
+    function intersect($other) {}
+
+    /**
+     * Returns true if the array is empty.
+     *
+     * @return bool
+     */
+    function isEmpty() {}
+
+    /**
+     * Returns true if the array is not empty.
+     */
+    function isNotEmpty() {}
+
+    /**
+     * Returns the last element matching the given predicate.
+     *
+     * @param callable $predicate[optional] ($value, $key) -> bool
+     * @return mixed
+     */
+    function last($predicate) {}
+
+    /**
+     * Returns last key of element, or null if the array does not contain element.
+     *
+     * @param mixed $element
+     * @return int|string|null
+     */
+    function lastIndexOf($element) {}
+
+    /**
+     * Returns a collection containing the results of applying the given transform function
+     * to each element in the original collection.
+     *
+     * @param callable $transform ($value, $key) -> $new_value
+     * @return Collection
+     */
+    function map($transform) {}
+
+    /**
+     * Returns a new collection with entries having the keys obtained by applying the transform function
+     * to each keys and values of this collection.
+     *
+     * @param callable $transform ($value, $key) -> $new_key
+     * @return Collection
+     */
+    function mapKeys($transform) {}
+
+    /**
+     * Populates the given destination collection with entries having the keys obtained by applying the
+     * transform function to each keys and values of this collections.
+     *
+     * @param Collection $destination
+     * @param callable $transform ($value, $key) -> $new_key
+     * @return Collection
+     */
+    function mapKeysTo($destination, $transform) {}
+
+    /**
+     * Applies the given transform function to each element of the original collection and appends the
+     * results to the given destination.
+     *
+     * @param Collection $destination
+     * @param callable $transform ($value, $key) -> $new_value
+     * @return Collection
+     */
+    function mapTo($destination, $transform) {}
+
+    /**
+     * Returns a new collection with entries having the keys of this collection and the values obtained
+     * by applying the transform function to each entry in this collection.
+     *
+     * @param callable $transform ($value, $key) -> $new_value
+     * @return Collection
+     */
+    function mapValues($transform) {}
+
+    /**
+     * Populates the given destination collection with entries having the keys of this collection and
+     * the values obtained by applying the transform function to each entry in this collection.
+     *
+     * @param Collection $destination
+     * @param callable $transform ($value, $key) -> $new_value
+     * @return Collection
+     */
+    function mapValuesTo($destination, $transform) {}
+
+    /**
+     * Returns the largest element or null if there are no elements.
+     *
+     * @return mixed
+     */
+    function max() {}
+
+    /**
+     * Returns the first element yielding the largest value of the given function or null if
+     * there are no elements.
+     *
+     * @param callable $selector ($value, $key) -> $new_value
+     * @return mixed
+     */
+    function maxBy($selector) {}
+
+    /**
+     * Returns the first element having the largest value according to the provided comparator or
+     * null if there are no elements.
+     *
+     * @param callable $comparator ($value_1, $value_2, $key_1, $key_2) -> int
+     * @return mixed
+     */
+    function maxWith($comparator) {}
+
+    /**
+     * Returns the largest element or null if there are no elements.
+     *
+     * @return mixed
+     */
+    function min() {}
+
+    /**
+     * Returns the first element yielding the smallest value of the given function or null if
+     * there are no elements.
+     *
+     * @param callable $selector ($value, $key) -> $new_value
+     * @return mixed
+     */
+    function minBy($selector) {}
+
+    /**
+     * Returns the first element having the smallest value according to the provided comparator or
+     * null if there are no elements.
+     *
+     * @param callable $comparator ($value_1, $value_2, $key_1, $key_2) -> int
+     * @return mixed
+     */
+    function minWith($comparator) {}
+
+    /**
+     * Returns a list containing all key-value pairs of the original collection except the ones contained
+     * in the given elements collection.
+     *
+     * @param array|Collection $elements
+     * @return Collection
+     */
+    function minus($elements) {}
+
+    /**
+     * Removes all key-value pairs contained in the given collection from this collection.
+     *
+     * @param array|Collection $elements
+     * @return void
+     */
+    function minusAssign($elements) {}
+
+    /**
+     * Returns a collection containing all entries of the original collection except those entries
+     * the keys of which are contained in the given keys collection.
+     *
+     * @param array|Collection $keys
+     * @return Collection
+     */
+    function minusKeys($keys) {}
+
+    /**
+     * Removes all entries the keys of which are contained in the given keys collection from this
+     * collection.
+     *
+     * @param array|Collection $keys
+     * @return void
+     */
+    function minusKeysAssign($keys) {}
+
+    /**
+     * Returns a list containing all elements of the original collection except the elements contained
+     * in the given elements collection.
+     *
+     * @param array|Collection $elements
+     * @return Collection
+     */
+    function minusValues($elements) {}
+
+    /**
+     * Removes all elements contained in the given elements collection from this collection.
+     *
+     * @param array|Collection $elements
+     * @return void
+     */
+    function minusValuesAssign($elements) {}
+
+    /**
+     * Returns true if no elements match the given predicate.
+     *
+     * @param callable $predicate ($value, $key) -> bool
+     * @return bool
+     */
+    function none($predicate) {}
+
+    /**
+     * @internal
+     */
+    function offsetUnset($offset) {}
+
+    /**
+     * @internal
+     */
+    function offsetSet($offset, $value) {}
+
+    /**
+     * @internal
+     */
+    function offsetGet($offset) {}
+
+    /**
+     * @internal
+     */
+    function offsetExists($offset) {}
+
+    /**
+     * Performs the given action on each element and returns the collection itself afterwards.
+     *
+     * @param callable $action ($value, $key) -> void
+     * @return Collection
+     */
+    function onEach($action) {}
+
+    /**
+     * Returns this collection if it's not null and the empty collection otherwise.
+     *
+     * @return Collection
+     */
+    function orEmpty() {}
+
+    /**
+     * Splits the original collection into pair of collections, where first collection contains
+     * elements for which predicate yielded true, while second collection contains elements for
+     * which predicate yielded false.
+     *
+     * @param callable $predicate ($value, $key) -> bool
+     * @return Pair
+     */
+    function partition($predicate) {}
+
+    /**
+     * Creates a new collection by replacing or adding entries to this collection from a given
+     * collection of key-value pairs.
+     *
+     * @param array|Collection $elements
+     * @return Collection
+     */
+    function plus($elements) {}
+
+    /**
+     * Appends or replaces all pairs from the given collection of pairs in this collection.
+     *
+     * @param array|Collection $elements
+     * @return void
+     */
+    function plusAssign($elements) {}
+
+    /**
+     * Returns a collection containing all elements of the original collection and then all elements
+     * of the given elements collection.
+     *
+     * @param array|Collection $elements
+     * @return Collection
+     */
+    function plusValues($elements) {}
+
+    /**
+     * Adds all elements of the given elements collection to this collection.
+     *
+     * @param array|Collection $elements
+     * @return void
+     */
+    function plusValuesAssign($elements) {}
+
+    /**
+     * Puts all the elements of the given collection into this collection.
+     *
+     * @param array|Collection $elements
+     * @return void
+     */
+    function putAll($elements) {}
+
+    /**
+     * Accumulates value starting with the first element and applying operation from left to right to
+     * current accumulator value and each element.
+     *
+     * @param $operation ($acc, $value, $key) -> $new_acc
+     * @return mixed
+     */
+    function reduce($operation) {}
+
+    /**
+     * Accumulates value starting with the last element and applying operation from right to left to
+     * each element and current accumulator value.
+     *
+     * @param $operation ($acc, $value, $key) -> $new_acc
+     * @return mixed
+     */
+    function reduceRight($operation) {}
+
+    /**
+     * Removes the entry for the specified key only if it is currently mapped to the specified value.
+     *
+     * @param int|string $key
+     * @param mixed $value[optional]
+     */
+    function remove($key, $value) {}
+
+    /**
+     * Removes all elements from this collection that match the given predicate.
+     *
+     * @param callable $predicate ($value, $key) -> bool
+     * @return bool
+     */
+    function removeAll($predicate) {}
+
+    /**
+     * Retains only elements of this collection that match the given predicate.
+     *
+     * @param callable $predicate ($value, $key) -> bool
+     * @return bool
+     */
+    function retainAll($predicate) {}
+
+    /**
+     * Reverses elements in the collection in-place.
+     *
+     * @return void
+     */
+    function reverse() {}
+
+    /**
+     * Returns a collection with elements in reversed order.
+     *
+     * @return Collection
+     */
+    function reversed() {}
+
+    /**
+     * Randomly shuffles elements in this collection.
+     *
+     * @return Collection
+     */
+    function shuffle() {}
+
+    /**
+     * Returns the single element matching the given predicate, or null if there is no or more than
+     * one matching element.
+     *
+     * @param callable $predicate ($value, $key) -> bool
+     * @return mixed
+     */
+    function single($predicate) {}
+
+    /**
+     * Returns a list containing elements at specified indices.
+     *
+     * @param array|Collection $indices
+     */
+    function slice($indices) {}
+
+    /**
+     * Sorts the collection in-place according to the natural order of its elements.
+     *
+     * @return void
+     */
+    function sort() {}
+
+    /**
+     * Sorts elements in the collection in-place according to natural sort order of the value returned
+     * by specified selector function.
+     *
+     * @param callable $selector ($value, $key) -> $new_value
+     * @return void
+     */
+    function sortBy($selector) {}
+
+    /**
+     * Sorts elements in the collection in-place descending according to natural sort order of the
+     * value returned by specified selector function.
+     *
+     * @param callable $selector ($value, $key) -> $new_value
+     * @return void
+     */
+    function sortByDescending($selector) {}
+
+    /**
+     * Sorts elements in the collection in-place descending according to their natural sort order.
+     *
+     * @return void
+     */
+    function sortDescending() {}
+
+    /**
+     * Sorts elements in the collection in-place according to the order specified with comparator.
+     *
+     * @param callable $comparator ($value_1, $value_2, $key_1, $key_2) -> int
+     * @return void
+     */
+    function sortWith($comparator) {}
+
+    /**
+     * Returns a collection of all elements sorted according to their natural sort order.
+     *
+     * @return Collection
+     */
+    function sorted() {}
+
+    /**
+     * Returns a collection of all elements sorted according to natural sort order of the
+     * value returned by specified selector function.
+     *
+     * @param callable $selector ($value, $key) -> $new_value
+     * @return Collection
+     */
+    function sortedBy($selector) {}
+
+    /**
+     * Returns a collection of all elements sorted descending according to natural sort order
+     * of the value returned by specified selector function.
+     *
+     * @param callable $selector ($value, $key) -> $new_value
+     * @return Collection
+     */
+    function sortedByDescending($selector) {}
+
+    /**
+     * Returns a collection of all elements sorted descending according to their natural sort order.
+     *
+     * @param callable $selector ($value, $key) -> $new_value
+     * @return Collection
+     */
+    function sortedDescending($selector) {}
+
+    /**
+     * Returns a collection of all elements sorted according to the specified comparator.
+     *
+     * @param callable $comparator ($value_1, $value_2, $key_1, $key_2) -> int
+     * @return Collection
+     */
+    function sortedWith($comparator) {}
+
+    /**
+     * Returns a collection containing first n elements.
+     *
+     * @param int $n
+     * @return Collection
+     */
+    function take($n) {}
+
+    /**
+     * Returns a list containing last n elements.
+     *
+     * @param int $n
+     * @return Collection
+     */
+    function takeLast($n) {}
+
+    /**
+     * Returns a collection containing last elements satisfying the given predicate.
+     *
+     * @param $predicate ($value, $key) -> bool
+     * @return Collection
+     */
+    function takeLastWhile($predicate) {}
+
+    /**
+     * Returns a collection containing first elements satisfying the given predicate.
+     *
+     * @param $predicate ($value, $key) -> bool
+     * @return Collection
+     */
+    function takeWhile($predicate) {}
+
+    /**
+     * Returns the array wrapped by this collection.
+     *
+     * @return array
+     */
+    function toArray() {}
+
+    /**
+     * Appends all elements to the given destination collection.
+     *
+     * @param Collection $destination
+     * @return Collection
+     */
+    function toCollection($destination) {}
+
+    /**
+     * Convert collection to a collection of pairs with keys being first and values being second.
+     *
+     * @return Collection
+     */
+    function toPairs() {}
+
+    /**
+     * Returns a collection containing all distinct elements from both collections.
+     *
+     * @param array|Collection $other
+     */
+    function union($other) {}
+}
\ No newline at end of file
diff --git a/stubs/Pair.php b/stubs/Pair.php
new file mode 100644
index 0000000..7ffb5ab
--- /dev/null
+++ b/stubs/Pair.php
@@ -0,0 +1,19 @@
+<?php
+
+/**
+ * A pair with two elements.
+ */
+class Pair
+{
+    public $first;
+
+    public $second;
+
+    /**
+     * Constructor.
+     *
+     * @param $first
+     * @param $second
+     */
+    function __construct($first, $second) {}
+}
\ No newline at end of file