Fix the issue of multiple rules existing in same earley state

Duplicate earley items should not exist concurrently in a given earley
state, but duplicate _rules_ should be allowed. This means that if two
earley items have the same rule, but different origins or positions,
they should be allowed to exist in the same state.

This will allow certain ambiguous grammars/sentences to successfully
parse that would otherwise fail.

lib/parser.js

@@ -45,7 +45,7 @@ function predict(tokens, states, i, j, grammar) {
     grammar.forEach((rule) => {
       let stateHasItem = states[i].filter((earleyItem) => {
         return earleyItem.rule === rule &&
-               curr.position === earleyItem.position;
+               curr.position === 0;//earleyItem.position;
       }).length > 0;
 
       if (!stateHasItem) {
@@ -55,10 +55,6 @@ function predict(tokens, states, i, j, grammar) {
           position: 0,
           origin  : i
         });
-
-        if (states[i].length > 100) {
-          throw new Error(JSON.stringify(states[i], null, 2));
-        }
       }
     });
   }
@@ -98,7 +94,8 @@ function complete(tokens, states, i, j) {
       if (earleyItem.rule[earleyItem.position] === curr.rule.lhs) {
         let stateHasItem = states[i].filter((ei) => {
           return ei.rule === earleyItem.rule &&
-                 ei.position === earleyItem.position + 1;
+                 ei.position === earleyItem.position + 1 &&
+                 ei.origin === earleyItem.origin;
         }).length > 0;
 
         if (stateHasItem) {
@@ -138,6 +135,9 @@ function dfs(states, tokens) {
     return best;
   }, null);
 
+  if (root == null) {
+    throw new SyntaxError(`Parsing error near '${tokens[0]}' `);
+  }
   if (root.origin !== tokens.length) {
     throw new SyntaxError(`Parsing error near '${tokens[root.origin]}' `);
   }