Re: diff lib

[marks]

I think we're doing two different kinds of diff. My library takes sequences `a` 
and `b` and returns a sequence of "spans" (i.e., insert/delete/replace a from 
to, b from to) that if applied to `a` would transform it into `b`. This is 
explained in the Python [difflib 
docs](https://docs.python.org/3/library/difflib.html).