Interface for Edit Distances.

An edit distance is a formal metric on the Kleene closure (X<sup>*</sup>) over an alphabet (X). Note, that a metric on a set S is a function d: [S * S] -> [0, INFINITY) such that the following hold for x,y,z in the set S:

• d(x,y) >= 0, non-negativity or separation axiom
• d(x,y) == 0, if and only if, x == y
• d(x,y) == d(y,x), symmetry, and
• d(x,z) <= d(x,y) + d(y,z), the triangle inequality

This is a BiFunction<CharSequence, CharSequence, R>. The apply method accepts a pair of CharSequence parameters and returns an R type similarity score.