Direct implementation of new operators.

Some operators are more easily defined in terms of the underlying automaton. For instance, the operator reverse(X) is the set of all strings Y such that the reversal of Y is in X. In case the operand X is constructed by means of standard regular expression operators it would be possible to define the reverse operator recursively in terms of the various forms that X can take; in FSA however X could be constructed by means of various user-defined operators as well. Therefore this approach is not applicable. However, the operation is trivial to define in terms of the underlying automaton: each of the transitions needs to be swapped, final states become start states and vice versa. The (simplified) definition is given as follows:

$$\begin{minipage}[t]{.9\textwidth}\begin{verbatim}rx(reverse... ...)\vert T]) :- reverse_trans(T0,T).\end{verbatim}\end{minipage}$$ (17)

As is typical in such definitions, the fsa_regex:rx predicate is used to construct an automaton for a given regular expression. The fsa_data module provides a consistent interface to the internal representation of automata. Its predicates can be used to select relevant parts of an automaton (such as start states, final states and transitions) and to construct automata on the basis of such parts.