Levenshtein Distance for the 3x4 Phone Keypads

This here should find a kind of Levenshtein distance for words, but it is based, as is my habit, on the layout of the keyboard, rather than being a naïve implementation. It is not generalised. This time, it is strictly for the phone.

It creates String#-, which is commutative, unlike the traditional -. So,

'man' - 'boy' = 'boy' - 'man'

Now:

'man' - 'men' = 2

And:

'man' - 'mbn' = 1

Which makes 'mbn' have a smaller distance from 'man', because they share a key on the phone (it could be a case of one pressing the key one time too many). However:

'man' - 'med' = 4

Which places 'men' closer to 'man', in case of bad spelling, than 'med'.

'man' - 'man' = 0

For there is no distance between the same two words.

An arbitrary/heuristic way to decide, sans interaction, which of the words in a list were intended, if all of them were missed, is to take the one that is closest, of those that are within < 3 in distance. If you get only one, use that.

The - operator has an unspecified time complexity that is huge. It is at least O(5n(n^2)) on the shortest string. Please use only with short strings, and sparingly. It has not been optimised for speed, just for the comfort of those who may have made the spelling errors (not even for the programmer).

TODO

1. Add accented characters to the keypad, when I get a text editor that permits it.