  We continue our study of open and closed languages. We investigate how the
properties of being open and closed are preserved under concatenation. We
investigate analogues, in formal languages, of the separation axioms in
topological spaces; one of our main results is that there is a clopen partition
separating two words if and only if the words commute. We show that we can
decide in quadratic time if the language specified by a DFA is closed, but if
the language is specified by an NFA, the problem is PSPACE-complete.
