  Let A be an alphabet and let F be a set of words with letters in A. We show
that the sum of all words with letters in A with no consecutive subwords in F,
as a formal power series in noncommuting variables, is the reciprocal of a
series with all coefficients 0, 1 or -1. We also explain how this result is
related to a result of Curtis Greene on lattices with M\"obius function 0, 1,
or -1.
