Interesting question-- whether or not the module Zaxo suggests does it for you, can we clarify what your intuitions are? From your example, I would guess that what matters to you is the number of operations required to turn one string into another (where character substitution and insertion are allowed as operations) as compared to the length of the strings (one-character strings 'a' and 'b' are, to me, less identical than 'aaaaaa' and 'aaaaba'). Any ideas?