diff --git a/src/engine/Value.js b/src/engine/Value.js
index <HASH>..<HASH> 100644
--- a/src/engine/Value.js
+++ b/src/engine/Value.js
@@ -5,6 +5,10 @@ function Value(value) {
     return _value;
   };
 
+  this.isGround = function isGround() {
+    return true;
+  };
+
   this.getVariables = function getVariables() {
     return [];
   };
diff --git a/src/engine/Variable.js b/src/engine/Variable.js
index <HASH>..<HASH> 100644
--- a/src/engine/Variable.js
+++ b/src/engine/Variable.js
@@ -6,6 +6,10 @@ function Variable(name) {
     return _name;
   };
 
+  this.isGround = function isGround() {
+    return false;
+  };
+
   this.getVariables = function getVariables() {
     return [_name];
   };
add isGround for value and Variable