diff --git a/FAILang/FAILangVisitor.cs b/FAILang/FAILangVisitor.cs
index d257971..f392ee4 100644
--- a/FAILang/FAILangVisitor.cs
+++ b/FAILang/FAILangVisitor.cs
@@ -87,7 +87,7 @@ public override IType VisitRelational([NotNull] FAILangParser.RelationalContext
                 return VisitBinary(context.binary(0));
             }
             var binaryNodes = context.binary();
-            var ops = context.GetTokens(context.op.Type);
+            var ops = context.relational_op();
 
             RelationalOperator[] opers = new RelationalOperator[ops.Length];
             for (int i = 0; i < opers.Length; i++)
@@ -107,10 +107,10 @@ public override IType VisitRelational([NotNull] FAILangParser.RelationalContext
                         opers[i] = RelationalOperator.LESS;
                         break;
                     case ">=":
-                        opers[i] = RelationalOperator.GR_EQUAL;
+                        opers[i] = RelationalOperator.GREATER_EQUAL;
                         break;
                     case "<=":
-                        opers[i] = RelationalOperator.LE_EQUAL;
+                        opers[i] = RelationalOperator.LESS_EQUAL;
                         break;
                 }
             }
@@ -149,6 +149,9 @@ public override IType VisitBinary([NotNull] FAILangParser.BinaryContext context)
                 case "^":
                     oper = BinaryOperator.EXPONENT;
                     break;
+                case "is":
+                    oper = BinaryOperator.IS;
+                    break;
             }
             return new BinaryOperatorExpression(oper, VisitBinary(binaryNodes[0]), VisitBinary(binaryNodes[1]));
         }
diff --git a/FAILang/Global.cs b/FAILang/Global.cs
index 951eaa7..bf4e25f 100644
--- a/FAILang/Global.cs
+++ b/FAILang/Global.cs
@@ -35,13 +35,14 @@ public static void ResetGlobals()
             "i",
             "true",
             "false",
-            "void",
+            "undefined",
             "lambda",
             "update",
             "memo",
             "self",
             "if",
-            "otherwise"
+            "otherwise",
+            "is"
         };
 
         public void LoadBuiltins(params IBuiltinProvider[] builtinProviders)
diff --git a/FAILang/Grammar/FAILang.g4 b/FAILang/Grammar/FAILang.g4
index 3d8ad1b..3a4ae1a 100644
--- a/FAILang/Grammar/FAILang.g4
+++ b/FAILang/Grammar/FAILang.g4
@@ -48,7 +48,7 @@ expression
 	;
 
 relational
-	: binary ( op=(EQ | NE | R_ARR | L_ARR | GE | LE) binary)*
+	: binary ( relational_op binary)*
 	;
 
 binary
@@ -56,6 +56,7 @@ binary
 	| <assoc=right> binary op=EXPONENT binary
 	| binary op=( MULTIPLY | DIVIDE | MODULO ) binary
 	| binary op=( PLUS | SUBTRACT ) binary
+	| binary op=IS binary
 	;
 
 prefix
@@ -114,6 +115,15 @@ union
 	: L_PAREN (expression VERT_LINE)+ expression R_PAREN
 	;
 
+relational_op
+	: EQ
+	| NE
+	| R_ARR
+	| L_ARR
+	| GE
+	| LE
+	;
+
 end
 	: SEMI_COLON
 	| 
@@ -268,6 +278,10 @@ OTHERWISE
 	: 'otherwise'
 	;
 
+IS
+	: 'is'
+	;
+
 NAME
 	:
 		( UPPERCASE
diff --git a/FAILang/Operator.cs b/FAILang/Operator.cs
index 457fba8..4a1651e 100644
--- a/FAILang/Operator.cs
+++ b/FAILang/Operator.cs
@@ -17,7 +17,8 @@ public enum BinaryOperator
         MULTIPLY,
         DIVIDE,
         MODULO,
-        EXPONENT
+        EXPONENT,
+        IS
     }
     public enum RelationalOperator
     {
@@ -25,8 +26,8 @@ public enum RelationalOperator
         NOT_EQUALS,
         GREATER,
         LESS,
-        GR_EQUAL,
-        LE_EQUAL
+        GREATER_EQUAL,
+        LESS_EQUAL
     }
     public enum UnaryOperator
     {
@@ -51,6 +52,8 @@ public static string ToDisplayString(this BinaryOperator op)
                     return "%";
                 case BinaryOperator.EXPONENT:
                     return "^";
+                case BinaryOperator.IS:
+                    return "is";
                 default:
                     return "";
             }
@@ -67,9 +70,9 @@ public static string ToDisplayString(this RelationalOperator op)
                     return ">";
                 case RelationalOperator.LESS:
                     return "<";
-                case RelationalOperator.GR_EQUAL:
+                case RelationalOperator.GREATER_EQUAL:
                     return ">=";
-                case RelationalOperator.LE_EQUAL:
+                case RelationalOperator.LESS_EQUAL:
                     return "<=";
                 default:
                     return "";
diff --git a/FAILang/Types/Number.cs b/FAILang/Types/Number.cs
index 70ad3e6..4eea8a8 100644
--- a/FAILang/Types/Number.cs
+++ b/FAILang/Types/Number.cs
@@ -32,9 +32,9 @@ public Number(Complex value)
             {RelationalOperator.EQUALS, OpEquals},
             {RelationalOperator.NOT_EQUALS, OpNotEquals},
             {RelationalOperator.GREATER, OpGreaterThan},
-            {RelationalOperator.GR_EQUAL, OpGreaterEqual},
+            {RelationalOperator.GREATER_EQUAL, OpGreaterEqual},
             {RelationalOperator.LESS, OpLessThan},
-            {RelationalOperator.LE_EQUAL, OpLessEqual}
+            {RelationalOperator.LESS_EQUAL, OpLessEqual}
         };
 
         public Dictionary<UnaryOperator, Func<IType>> UnaryOperators => new Dictionary<UnaryOperator, Func<IType>>()
diff --git a/FAILang/Types/Unevaluated/BinaryOperatorExpression.cs b/FAILang/Types/Unevaluated/BinaryOperatorExpression.cs
index 669326c..a1345ac 100644
--- a/FAILang/Types/Unevaluated/BinaryOperatorExpression.cs
+++ b/FAILang/Types/Unevaluated/BinaryOperatorExpression.cs
@@ -56,6 +56,11 @@ public IType Evaluate(Dictionary<string, IType> lookups)
                 return eRight;
 
             // Operate
+            if (op == BinaryOperator.IS)
+            {
+                return left.Equals(right) ? MathBool.TRUE : MathBool.FALSE;
+            }
+
             if (left == Undefined.instance || right == Undefined.instance)
                     return Undefined.instance;