substitutions in dl_distance time complexity

[raphael-sch]

If you run dl_distance in a time critical loop and use substitutions, you don't want to import defaultdict and iterate through all subs every time. Instead dl_distance should just take a dict as substitution parameter and let 'key in dict' decide to return value or 1.

[nadvornix]

What do you think about this way of fixing it?

[raphael-sch]

yes in general this is how I would do it. In #5 i suggest a few changes to your commit.