diff --git a/Context.php b/Context.php
index a4b3acc..9f49516 100644
--- a/Context.php
+++ b/Context.php
@@ -112,7 +112,7 @@ public function offsetGet($id)
         }
 
         if (true === is_callable($value)) {
-            if (true  === is_string($value) &&
+            if (true === is_string($value) &&
                 false === in_array(strtolower($value), get_defined_functions()['user'])) {
                 return $value;
             }
diff --git a/Model/Operator.php b/Model/Operator.php
index 75742f3..e219cb1 100644
--- a/Model/Operator.php
+++ b/Model/Operator.php
@@ -47,9 +47,7 @@
  * @copyright  Copyright © 2007-2017 Hoa community
  * @license    New BSD License
  */
-class          Operator
-    extends    Ruler\Model\Bag\Context
-    implements Visitor\Element
+class Operator extends Ruler\Model\Bag\Context implements Visitor\Element
 {
     /**
      * Lazy evaluation should break.
diff --git a/Test/Unit/DynamicCallable.php b/Test/Unit/DynamicCallable.php
index 6b0378a..0ab35e0 100644
--- a/Test/Unit/DynamicCallable.php
+++ b/Test/Unit/DynamicCallable.php
@@ -53,7 +53,8 @@ class DynamicCallable extends Test\Unit\Suite
     public function case_is_a_xcallable()
     {
         $this
-            ->when($result = new SUT(function () {}))
+            ->when($result = new SUT(function () {
+            }))
             ->then
                 ->object($result)
                     ->isInstanceOf(Consistency\Xcallable::class);
diff --git a/Test/Unit/Visitor/Asserter.php b/Test/Unit/Visitor/Asserter.php
index 65b9087..367dde3 100644
--- a/Test/Unit/Visitor/Asserter.php
+++ b/Test/Unit/Visitor/Asserter.php
@@ -404,7 +404,8 @@ public function case_set_operator()
             ->given(
                 $asserter     = new SUT(),
                 $oldOperators = $asserter->getOperators(),
-                $operator     = function () {}
+                $operator     = function () {
+                }
             )
             ->when($result = $asserter->setOperator('_foo_', $operator))
             ->then
@@ -418,14 +419,34 @@ public function case_set_operator()
                     ->isEqualTo(xcallable($operator));
     }
 
+    public function case_set_operator_insensitive()
+    {
+        $this
+            ->given(
+                $asserter     = new SUT(),
+                $operator     = function () {
+                }
+            )
+            ->when($result = $asserter->setOperator('_FΛO_', $operator))
+            ->then
+                ->boolean($asserter->operatorExists('_FΛO_'))
+                    ->isTrue()
+                ->boolean($asserter->operatorExists('_fλo_'))
+                    ->isTrue()
+                ->object($asserter->getOperator('_fλo_'))
+                    ->isEqualTo(xcallable($operator));
+    }
+
     public function case_set_operator_overwrite()
     {
         $this
             ->given(
                 $asserter = new SUT(),
-                $asserter->setOperator('_foo_', function () {}),
+                $asserter->setOperator('_foo_', function () {
+                }),
                 $oldOperators = $asserter->getOperators(),
-                $operator = function () {}
+                $operator = function () {
+                }
             )
             ->when($result = $asserter->setOperator('_foo_', $operator))
             ->then
@@ -444,7 +465,8 @@ public function case_operator_exists()
         $this
             ->given(
                 $asserter = new SUT(),
-                $asserter->setOperator('_foo_', function () {})
+                $asserter->setOperator('_foo_', function () {
+                })
             )
             ->when($result = $asserter->operatorExists('_foo_'))
             ->then
@@ -467,7 +489,8 @@ public function case_get_operator()
         $this
             ->given(
                 $asserter = new SUT(),
-                $operator = function () {},
+                $operator = function () {
+                },
                 $asserter->setOperator('_foo_', $operator)
             )
             ->when($result = $asserter->getOperator('_foo_'))
diff --git a/Visitor/Asserter.php b/Visitor/Asserter.php
index ca15d60..c8d2c3e 100644
--- a/Visitor/Asserter.php
+++ b/Visitor/Asserter.php
@@ -77,19 +77,43 @@ public function __construct(Ruler\Context $context = null)
             $this->setContext($context);
         }
 
-        $this->setOperator('and', function ($a = false, $b = false) { return $a && $b; });
-        $this->setOperator('or',  function ($a = false, $b = false) { return $a || $b; });
-        $this->setOperator('xor', function ($a, $b) { return (bool) ($a ^ $b); });
-        $this->setOperator('not', function ($a) { return !$a; });
-        $this->setOperator('=',   function ($a, $b) { return $a == $b; });
-        $this->setOperator('is',  $this->getOperator('='));
-        $this->setOperator('!=',  function ($a, $b) { return $a != $b; });
-        $this->setOperator('>',   function ($a, $b) { return $a >  $b; });
-        $this->setOperator('>=',  function ($a, $b) { return $a >= $b; });
-        $this->setOperator('<',   function ($a, $b) { return $a <  $b; });
-        $this->setOperator('<=',  function ($a, $b) { return $a <= $b; });
-        $this->setOperator('in',  function ($a, array $b) { return in_array($a, $b); });
-        $this->setOperator('sum', function () { return array_sum(func_get_args()); });
+        $this->setOperator('and', function ($a = false, $b = false) {
+            return $a && $b;
+        });
+        $this->setOperator('or', function ($a = false, $b = false) {
+            return $a || $b;
+        });
+        $this->setOperator('xor', function ($a, $b) {
+            return (bool) ($a ^ $b);
+        });
+        $this->setOperator('not', function ($a) {
+            return !$a;
+        });
+        $this->setOperator('=', function ($a, $b) {
+            return $a == $b;
+        });
+        $this->setOperator('is', $this->getOperator('='));
+        $this->setOperator('!=', function ($a, $b) {
+            return $a != $b;
+        });
+        $this->setOperator('>', function ($a, $b) {
+            return $a > $b;
+        });
+        $this->setOperator('>=', function ($a, $b) {
+            return $a >= $b;
+        });
+        $this->setOperator('<', function ($a, $b) {
+            return $a < $b;
+        });
+        $this->setOperator('<=', function ($a, $b) {
+            return $a <= $b;
+        });
+        $this->setOperator('in', function ($a, array $b) {
+            return in_array($a, $b);
+        });
+        $this->setOperator('sum', function () {
+            return array_sum(func_get_args());
+        });
         $this->setOperator('matches', function ($subject, $pattern) {
             $escapedPattern = preg_replace('/(?<!\\\)`/', '\`', $pattern);
 
@@ -499,7 +523,7 @@ public function getContext()
      */
     public function setOperator($operator, $callable)
     {
-        $this->_operators[$operator] = $callable;
+        $this->_operators[mb_strtolower($operator)] = $callable;
 
         return $this;
     }
@@ -512,7 +536,7 @@ public function setOperator($operator, $callable)
      */
     public function operatorExists($operator)
     {
-        return true === array_key_exists($operator, $this->_operators);
+        return true === array_key_exists(mb_strtolower($operator), $this->_operators);
     }
 
     /**
@@ -527,6 +551,8 @@ public function getOperator($operator)
             return null;
         }
 
+        $operator = mb_strtolower($operator);
+
         $handle = &$this->_operators[$operator];
 
         if (!($handle instanceof Consistency\Xcallable)) {
diff --git a/Visitor/Compiler.php b/Visitor/Compiler.php
index 99fe953..b14799e 100644
--- a/Visitor/Compiler.php
+++ b/Visitor/Compiler.php
@@ -100,7 +100,7 @@ public function visit(Visitor\Element $element, &$handle = null, $eldnah = null)
 
                 $out .= '(' . "\n";
             } else {
-                $out       .= 'func(' . "\n" . $_ . '    ';
+                $out .= 'func(' . "\n" . $_ . '    ';
                 $_handle[]  = '\'' . $name . '\'';
             }
 
@@ -173,7 +173,7 @@ protected function visitContext(Ruler\Model\Bag\Context $context, &$handle, $eld
             ++$this->_indentation;
 
             $value  = $dimension[Ruler\Model\Bag\Context::ACCESS_VALUE];
-            $out   .= "\n" . $_ . '    ->';
+            $out .= "\n" . $_ . '    ->';
 
             switch ($dimension[Ruler\Model\Bag\Context::ACCESS_TYPE]) {
                 case Ruler\Model\Bag\Context::ARRAY_ACCESS: