UDCode - Does a set of code words form a uniquely decodable code?

A code is a set of strings, called the _code words_.  A code is
"uniquely decodable" if any string S that is a concatenation of
code words is so in _exactly one way_.

For example, the code ("ab", "abba", "b") is _not_ uniquely
decodable, because "abba" . "b" eq "ab" . "b" . "ab".  But the code
("a", "ab", "abb") _is_ uniquely decodable, because there is no such
pair of sequences of code words.

UDcode contains a function "is_udcode" that tells whether a set of
code words is a uniquely decodable code, and a function "ud_pair" that
returns two sequences of code words whose concatenations are the same,
if there is such a pair.

Author: Mark Jason Dominus (mjd@plover.com)

License: Public domain
