Simple Prolog Constraints

Because finite-state automata are defined by Prolog clauses, it is very simple to attach Prolog constraints. For example, suppose that for a certain application it is useful to consider two subclasses of the alphabet, namely V (vowels) and C (consonants), we could then use the following technique to define the sequences C^*V^*C^*:

vowel(a). vowel(e). vowel(i). vowel(o). vowel(u). cons(b). cons(c). cons(d). cons(f). cons(g). cons(h). cons(j). cons(k). cons(l). cons(m). cons(n). cons(p). cons(q). cons(r). cons(s). cons(t). cons(v). cons(w). cons(y). cons(z). start(0). final(2). jump(0,1). jump(1,2). trans(0,C,0) :- cons(C). trans(1,V,1) :- vowel(V). trans(2,C,2) :- cons(C).

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I only allow constraints for which Prolog's built-in search procedure terminates. These Prolog constraints therefore do not increase the formal power of the formalism, although they are useful to define automata in a convenient and concise way.