Fixed tree-construction semantics: Alias now overrides the "?rule" operator

Breaking change!!

lark/parse_tree_builder.py

@@ -123,7 +123,7 @@ class ParseTreeBuilder:
                         raise Exception("Rule %s is marked for expansion (it starts with an underscore) and isn't allowed to have aliases (alias=%s)" % (origin, alias))
 
                 wrapper_chain = filter(None, [
-                    expand1 and Expand1,
+                    (expand1 and not alias) and Expand1,
                     create_token and Factory(TokenWrapper, create_token),
                     create_rule_handler(expansion, keep_all_tokens, filter_out),
                     self.propagate_positions and PropagatePositions,

tests/test_parser.py

@@ -78,7 +78,13 @@ class TestParsers(unittest.TestCase):
                  """)
 
         r = g.parse('x')
-        self.assertEqual( r.children[0].data, "b" )
+        self.assertEqual( r.children[0].data, "c" )
+
+        g = Lark("""start: a
+                    ?a: B -> c
+                    B: "x"
+                 """)
+        self.assertEqual( r.children[0].data, "c" )
 
 
         g = Lark("""start: a