True when Automaton is the term graph of Term
written out as an explicit automaton, and Term is the term
its start state denotes. Which way the predicate runs is decided by Automaton:
it answers Automaton if that is unbound, and Term
if it is not.
Automaton is a compound whose argument I is the
colour of state I, and state 1 is the start state.
An argument that is a compound gives the state one successor per
argument of that compound, and the argument is the number of the state
the successor goes to. An argument that is not a compound makes the
state a sink holding that value. So a leaf is a state rather
than an argument, which is why nothing in an argument position is ever
anything but a state number:
?- term_automaton(f(g(a),g(a)), A).
A = automaton(f(2, 3), g(4), g(4), a).
The translation is faithful in both directions: one state per
physically distinct compound cell of Term and one per
distinct leaf, sharing what Term already shares and
collapsing nothing.
Term may be cyclic, and so may Automaton:
?- X = f(X), term_automaton(X, A).
A = automaton(f(1)).
The functor name of Automaton is not looked at when it is
read; like arg/3,
only the arity matters. The name used when it is written is always automaton.
Note that the value of a sink can never be a compound, so a state label
that is a compound has to be taken apart into states of its own.
