Fix #50 on Github.

Context.php

@@ -113,8 +113,14 @@ public function offsetGet ( $id ) {
         if($value instanceof DynamicCallable)
             return $value($this);
 
-        if(true === is_callable($value))
+        if(true === is_callable($value)) {
+
+            if(   true  === is_string($value)
+               && false === in_array(strtolower($value), get_defined_functions()['user']))
+                return $value;
+
             $value = $this->_data[$id] = $value($this);
+        }
 
         return $value;
     }

Test/Unit/Bug/Github50.php

@@ -0,0 +1,70 @@
+<?php
+
+/**
+ * Hoa
+ *
+ *
+ * @license
+ *
+ * New BSD License
+ *
+ * Copyright © 2007-2015, Ivan Enderlin. All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *     * Redistributions of source code must retain the above copyright
+ *       notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above copyright
+ *       notice, this list of conditions and the following disclaimer in the
+ *       documentation and/or other materials provided with the distribution.
+ *     * Neither the name of the Hoa nor the names of its contributors may be
+ *       used to endorse or promote products derived from this software without
+ *       specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDERS AND CONTRIBUTORS BE
+ * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ */
+
+namespace Hoa\Ruler\Test\Unit\Bug;
+
+use Hoa\Test;
+use Hoa\Ruler as LUT;
+
+/**
+ * Class \Hoa\Ruler\Test\Unit\Bug\Github50.
+ *
+ * See https://github.com/hoaproject/Ruler/issues/50.
+ *
+ * @author     Ivan Enderlin <[email protected]>
+ * @copyright  Copyright © 2007-2015 Ivan Enderlin.
+ * @license    New BSD License
+ */
+
+class Github50 extends Test\Unit\Suite {
+
+    public function case_bug ( ) {
+
+        $this
+            ->given(
+                $ruler               = new LUT(),
+                $rule                = 'variable',
+                $context             = new LUT\Context(),
+                $context['variable'] = 'file'
+            )
+            ->when(function ( ) use ( $ruler, $rule, $context ) {
+
+                $ruler->assert($rule, $context);
+            })
+                ->error()
+                    ->notExists();
+    }
+}