A DOUBLET puzzle consists of a list of words, all of the same length. The first and last words in the list are given, and are called
Anchor Words. Each of the intervening words is defined by a clue, just as in a crossword puzzle, but in addition it is formed from the
previous word in the list by the application of one of a set of
Doublet Rules. Currently, there are three rules, giving rise to a family
of three types of DOUBLET puzzles.