Add cotan function for infinite decimal.

Author: codisart

src/InfiniteDecimal.php

@@ -281,6 +281,14 @@ public function tan($scale = null)
         );
     }
 
+    public function cotan($scale = null)
+    {
+        throw new \DomainException(($this === self::$pInf) ?
+            "Cotangent function hasn't limit in the positive infinite." :
+            "Cotangent function hasn't limit in the negative infinite."
+        );
+    }
+
     /**
      * @return boolean
      */

tests/InfiniteDecimal/InfiniteDecimalCotanTest.php

@@ -0,0 +1,27 @@
+<?php
+
+use Litipk\BigNumbers\InfiniteDecimal as InfiniteDecimal;
+
+/**
+ * @group cotan
+ */
+class InfiniteDecimalCotanTest extends PHPUnit_Framework_TestCase
+{
+    /**
+     * @expectedException \DomainException
+     * @expectedExceptionMessage Cotangent function hasn't limit in the positive infinite.
+     */
+    public function testFinitePositiveInfiniteCotan()
+    {
+        InfiniteDecimal::getPositiveInfinite()->cotan();
+    }
+
+    /**
+     * @expectedException \DomainException
+     * @expectedExceptionMessage Cotangent function hasn't limit in the negative infinite.
+     */
+    public function testFiniteNegativeInfiniteCotan()
+    {
+        InfiniteDecimal::getNegativeInfinite()->cotan();
+    }
+}