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Logtalk - Object oriented extension to Prolog
Release 2.29.4

Copyright (c) 1998-2007 Paulo Moura.  All Rights Reserved.
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% start by loading the example:

| ?- logtalk_load(metainterpreters(loader)).
...


% direct call of p/1:

| ?- database::p(X).

X = 1 ;
X = 2 
yes


% solver - a simple meta-interpreter for pure Prolog:

| ?- database::solve(p(X)).

X = 1 ;
X = 2 
yes

| ?- database::proof_tree(p(X), Tree).

X = 1
Tree = p(1):- (q(1, a):- (s(1):-true), (t(1, a):-true)), (r(a):-true) ;
X = 2
Tree = p(2):- (q(2, b):- (s(2):-true), (t(2, b):-true)), (r(b):-true) 
yes


% tracer - a simple meta-interpreter for tracing goal proofs using pure Prolog:

| ?- database::trace(p(X)).
1 call: p(_G180)
2 call: q(_G180, _G316)
3 call: s(_G180)
3 exit: s(1)
3 call: t(1, _G316)
3 exit: t(1, a)
2 exit: q(1, a)
2 call: r(a)
2 exit: r(a)
1 exit: p(1)

X = 1 ;
1 redo: p(1)
2 redo: r(a)
2 fail: r(a)
2 redo: q(1, a)
3 redo: t(1, a)
3 fail: t(1, _G316)
3 redo: s(1)
3 exit: s(2)
3 call: t(2, _G316)
3 exit: t(2, b)
2 exit: q(2, b)
2 call: r(b)
2 exit: r(b)
1 exit: p(2)

X = 2 ;
1 redo: p(2)
2 redo: r(b)
2 fail: r(b)
2 redo: q(2, b)
3 redo: t(2, b)
3 fail: t(2, _G316)
3 redo: s(2)
3 exit: s(3)
3 call: t(3, _G316)
3 fail: t(3, _G316)
3 redo: s(3)
3 fail: s(_G180)
2 fail: q(_G180, _G316)
1 fail: p(_G180)

no


% another example: expert system rules:

| ?- rules::prove(weather(Wheather)).

Wheather = raining 
yes

| ?- rules::prove(goto(Where)).

Where = cinema 
yes