diff --git a/mtl/parser.py b/mtl/parser.py
index 4602663..29f0c97 100644
--- a/mtl/parser.py
+++ b/mtl/parser.py
@@ -1,11 +1,16 @@
 # -*- coding: utf-8 -*-
 import operator as op
 from functools import partialmethod, reduce
+from typing import TypeVar
 
 from parsimonious import Grammar, NodeVisitor
 from mtl import ast
 from mtl import sugar
 
+
+MTL = TypeVar("MTL")
+
+
 MTL_GRAMMAR = Grammar(u'''
 phi = (neg / paren_phi / next / bot / top
      / xor_outer / iff_outer / implies_outer / and_outer / or_outer