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.

'is_udcode'
'is_udcode(@words)' returns true if and only if the specified code is
uniquely decodable.