Fix function redeclare fatal error

src/functions.php

@@ -1,34 +1,42 @@
 <?php
 namespace Ds;
 
-/**
- * Create a new Seq from an iterable.
- */
-function seq(iterable $values = []): Seq
-{
-    return new Seq($values);
+if (!function_exists('Ds\\seq')) {
+    /**
+     * Create a new Seq from an iterable.
+     */
+    function seq(iterable $values = []): Seq
+    {
+        return new Seq($values);
+    }
 }
 
-/**
- * Create a new Map from an iterable of key => value pairs.
- */
-function map(iterable $values = []): Map
-{
-    return new Map($values);
+if (!function_exists('Ds\\map')) {
+    /**
+     * Create a new Map from an iterable of key => value pairs.
+     */
+    function map(iterable $values = []): Map
+    {
+        return new Map($values);
+    }
 }
 
-/**
- * Create a new Set from an iterable.
- */
-function set(iterable $values = []): Set
-{
-    return new Set($values);
+if (!function_exists('Ds\\set')) {
+    /**
+     * Create a new Set from an iterable.
+     */
+    function set(iterable $values = []): Set
+    {
+        return new Set($values);
+    }
 }
 
-/**
- * Create a new Heap from an iterable with optional comparator.
- */
-function heap(iterable $values = [], ?callable $comparator = null): Heap
-{
-    return new Heap($values, $comparator);
+if (!function_exists('Ds\\heap')) {
+    /**
+     * Create a new Heap from an iterable with optional comparator.
+     */
+    function heap(iterable $values = [], ?callable $comparator = null): Heap
+    {
+        return new Heap($values, $comparator);
+    }
 }