  Let S be a finite set of words over an alphabet Sigma. The set S is said to
be complete if every word w over the alphabet Sigma is a factor of some element
of S*, i.e. w belongs to Fact(S*). Otherwise if S is not complete, we are
interested in finding bounds on the minimal length of words in Sigma* which are
not elements of Fact(S*) in terms of the maximal length of words in S.

Formal Languages and Automata Theory