BUGFIX: Internally repetitive rules are now handled silently (Issue #60)

lark/load_grammar.py

@@ -196,7 +196,7 @@ class SimplifyRule_Visitor(Visitor):
                     tree.data = 'expansions'
                     tree.children = [self.visit(T('expansion', [option if i==j else other
                                                                 for j, other in enumerate(tree.children)]))
-                                     for option in child.children]
+                                     for option in set(child.children)]
                     break
             else:
                 break

tests/test_parser.py

@@ -796,6 +796,16 @@ def _make_parser_test(LEXER, PARSER):
             self.assertEqual(tree.children, ['a', 'A'])
 
 
+        def test_twice_empty(self):
+            g = """!start: [["A"]]
+                """
+            l = _Lark(g)
+            tree = l.parse('A')
+            self.assertEqual(tree.children, ['A'])
+
+            tree = l.parse('')
+            self.assertEqual(tree.children, [])
+
         def test_undefined_ignore(self):
             g = """!start: "A"
 
@@ -1016,6 +1026,7 @@ def _make_parser_test(LEXER, PARSER):
             self.assertEqual(tree.children, [])
 
 
+
         @unittest.skipIf(LEXER==None, "Scanless doesn't support regular expressions")
         def test_regex_escaping(self):
             g = _Lark("start: /[ab]/")