Use deque.popleft() instead of list.pop(0) (nltk#3105)

Author: catleeball

nltk/grammar.py

@@ -69,6 +69,7 @@
 "expanding" *lhs* to *rhs* in *tree*.
 """
 import re
+from collections import deque
 from functools import total_ordering
 
 from nltk.featstruct import SLASH, TYPE, FeatDict, FeatStruct, FeatStructReader
@@ -771,15 +772,15 @@ def remove_unitary_rules(cls, grammar):
         lexical
         """
         result = []
-        unitary = []
+        unitary = deque([])
         for rule in grammar.productions():
             if len(rule) == 1 and rule.is_nonlexical():
                 unitary.append(rule)
             else:
                 result.append(rule)
 
         while unitary:
-            rule = unitary.pop(0)
+            rule = unitary.popleft()
             for item in grammar.productions(lhs=rule.rhs()[0]):
                 new_rule = Production(rule.lhs(), item.rhs())
                 if len(new_rule) != 1 or new_rule.is_lexical():