1diff --git a/src/Parser/Parser.ts b/src/Parser/Parser.ts
 2index b8d2b89..2147165 100644
 3--- a/src/Parser/Parser.ts
 4+++ b/src/Parser/Parser.ts
 5@@ -100,6 +100,11 @@ const enum ReduceActions {
 6   REDUCE_LAST_THREE_B
 7 }
 8 
 9+/**
10+ * Productions is used to look up both the number to use when reducing the stack (productions[x][1]) and the semantic
11+ * value that will replace the tokens in the stack (productions[x][0]).
12+ * @type {Array}
13+ */
14 let productions = [];
15 productions[ReduceActions.NO_ACTION] = 0;
16 productions[ReduceActions.RETURN_LAST] = [3, 2];
17@@ -1135,8 +1140,8 @@ let Parser = (function () {
18     parse: function parse(input) {
19       let self = this,
20         stack = [0],
21-        semanticValueStack = [null], // semantic value stack
22-        locationStack = [], // location stack
23+        semanticValueStack = [null],
24+        locationStack = [],
25         table = this.table,
26         yytext = '',
27         yylineno = 0,
28@@ -1146,9 +1151,6 @@ let Parser = (function () {
29         EOF = 1;
30 
31       let args = locationStack.slice.call(arguments, 1);
32-
33-      //this.reductionCount = this.shiftCount = 0;
34-
35       let lexer = Object.create(this.lexer);
36       let sharedState = {
37         yy: {
38@@ -1210,7 +1212,6 @@ let Parser = (function () {
39           _$: undefined
40         },
41         p,
42-        len,
43         newState,
44         expected;
45       while (true) {
46@@ -1343,19 +1344,21 @@ let Parser = (function () {
47             break;
48 
49           case LexActions.REDUCE: // Reduce
50-            len = this.productions[action[1]][1];
51+            let currentProduction = this.productions[action[1]];
52+
53+            let lengthToReduceStackBy = currentProduction[1];
54 
55             // perform semantic action
56-            yyval.$ = semanticValueStack[semanticValueStack.length - len]; // default to $$ = $1
57+            yyval.$ = semanticValueStack[semanticValueStack.length - lengthToReduceStackBy]; // default to $$ = $1
58             // default location, uses first token for firsts, last for lasts
59             yyval._$ = {
60-              first_line: locationStack[locationStack.length - (len || 1)].first_line,
61+              first_line: locationStack[locationStack.length - (lengthToReduceStackBy || 1)].first_line,
62               last_line: locationStack[locationStack.length - 1].last_line,
63-              first_column: locationStack[locationStack.length - (len || 1)].first_column,
64+              first_column: locationStack[locationStack.length - (lengthToReduceStackBy || 1)].first_column,
65               last_column: locationStack[locationStack.length - 1].last_column
66             };
67             if (ranges) {
68-              yyval._$.range = [locationStack[locationStack.length - (len || 1)].range[0], locationStack[locationStack.length - 1].range[1]];
69+              yyval._$.range = [locationStack[locationStack.length - (lengthToReduceStackBy || 1)].range[0], locationStack[locationStack.length - 1].range[1]];
70             }
71             result = this.performAction.apply(yyval, [yytext, sharedState.yy, action[1], semanticValueStack].concat(args));
72 
73@@ -1364,14 +1367,14 @@ let Parser = (function () {
74             }
75 
76             // pop off stack
77-            if (len) {
78-              stack = stack.slice(0, -1 * len * 2);
79-              semanticValueStack = semanticValueStack.slice(0, -1 * len);
80-              locationStack = locationStack.slice(0, -1 * len);
81+            if (lengthToReduceStackBy) {
82+              stack = stack.slice(0, -1 * lengthToReduceStackBy * 2);
83+              semanticValueStack = semanticValueStack.slice(0, -1 * lengthToReduceStackBy);
84+              locationStack = locationStack.slice(0, -1 * lengthToReduceStackBy);
85             }
86 
87             // push non-terminal (reduce)
88-            stack.push(this.productions[action[1]][0]);
89+            stack.push(currentProduction[0]);
90             semanticValueStack.push(yyval.$);
91             locationStack.push(yyval._$);
92             newState = table[stack[stack.length - 2]][stack[stack.length - 1]];