Fix for slices getting reduce to single getitem

examples/advanced/python3.lark

@@ -134,7 +134,7 @@ AWAIT: "await"
 tuplelist_comp: (test|star_expr) (comp_for | ("," (test|star_expr))+ [","] | ",")
 ?subscriptlist: subscript
               | subscript (("," subscript)+ [","] | ",") -> subscript_tuple
-subscript: test | [test] ":" [test] [sliceop]
+subscript: test | ([test] ":" [test] [sliceop]) -> slice
 sliceop: ":" [test]
 exprlist: (expr|star_expr)
         | (expr|star_expr) (("," (expr|star_expr))+ [","]|",") -> exprlist_tuple

lark/tree_matcher.py

@@ -51,7 +51,7 @@ def _match(term, token):
         return token.data == name
     elif isinstance(token, Token):
         return term == Terminal(token.type)
-    assert False
+    assert False, (term, token)
 
 
 def make_recons_rule(origin, expansion, old_expansion):