How to generate k+1 length strings from a list of k length strings?

[MTD]

Jon Clements wrote:
> Are you asking the question, "Which pairs of strings have one character
> different in each?", or "Which pairs of strings have a substring of
> len(string) - 1 in common?".
>
> Jon.

I imagine it's the former because the latter is trivially easy, I mean
_really_ trivially easy.