diff --git a/src/NXP/Classes/Calculator.php b/src/NXP/Classes/Calculator.php
index 4c878b0..2861125 100644
--- a/src/NXP/Classes/Calculator.php
+++ b/src/NXP/Classes/Calculator.php
@@ -44,7 +44,11 @@ class Calculator
                 if(array_key_exists($variable, $variables)) {
                     $value = $variables[$variable];
                 } elseif (array_key_exists($variable, $midresults)) { //增加对中间结果的递归计算
-                    $value = $oExecutor->execute($midresults[$variable]['formula']);
+                    if(isset($midresults[$variable]['value'])) { //增加对中间结果的使用
+                        $value = $midresults[$variable]['value'];
+                    } else {
+                        $value = $oExecutor->execute($midresults[$variable]['formula']);
+                    }
                 } else {
                     throw new UnknownVariableException();
                 }
diff --git a/src/NXP/Classes/Token/TokenFunction.php b/src/NXP/Classes/Token/TokenFunction.php
index 23f64bd..4e479ef 100644
--- a/src/NXP/Classes/Token/TokenFunction.php
+++ b/src/NXP/Classes/Token/TokenFunction.php
@@ -35,6 +35,9 @@ class TokenFunction extends AbstractContainerToken implements InterfaceFunction
             array_push($args, array_pop($stack)->getValue());
         }
         $result = call_user_func_array($function, $args);
+        if(is_nan($result)) { //增加把不合法计算表达式的值置0
+            $result = 0.0;
+        }
 
         return new TokenNumber($result);
     }
diff --git a/src/NXP/MathExecutor.php b/src/NXP/MathExecutor.php
index b88080d..d85c73d 100644
--- a/src/NXP/MathExecutor.php
+++ b/src/NXP/MathExecutor.php
@@ -109,6 +109,9 @@ class MathExecutor
         $this->tokenFactory->addFunction('elt', function ($arg1, $arg2, $arg3, $arg4) {
             return $arg4 <= $arg3 ? $arg2 : $arg1;
         }, 4);
+        $this->tokenFactory->addFunction('eq', function ($arg1, $arg2, $arg3, $arg4) {
+            return $arg4 == $arg3 ? $arg2 : $arg1;
+        }, 4);
         $this->addComplexFunc();
     }
 
@@ -278,7 +281,7 @@ class MathExecutor
      * @param  int          $places   Count of arguments
      * @return MathExecutor
      */
-    public function addFunction($name, $function = null, $places = 1)
+    public function addFunction($name, callable $function = null, $places = 1)
     {
         $this->tokenFactory->addFunction($name, $function, $places);