!TITLE Prolog In Prolog
!KEY
This example shows how simple it is to write a Prolog  interpreter  in  Prolog,
and  illustrates  the  use of a variable goal.  In this mini-interpreter, goals
and clauses are represented as ordinary Prolog data structures (i.e.    terms).
Terms  representing  clauses are specified using the unary predicate my_clause,
e.g.

            my_clause( (grandparent(X,Z):-parent(X,Y),parent(Y,Z)) ).

A unit clause will be represented by a term such as

            my_clause( (parent(john,mary) :- true) ).

The mini-interpreter consists of four clauses:

    | execute(true) :-!.
    | execute((P,Q)) :- !, execute(P), execute(Q).
    | execute(P) :- my_clause((P:-Q)), execute(Q).
    | execute(P) :- P.

The last clause enables the mini-interpreter to cope  with  calls  to  ordinary
Prolog predicates, e.g.  evaluable predicates.