/*===========================================================================

  In the following A1 is the answer of s1, A21 and A22 the answers of s2(X,Y),
  AaY is the answer of s2(a,Y), and A31 and A32 the answers of s3(Y).
  To understand the order of some of the following remember that delay
  elements are interned in the opposite order than they appear in the
  interned delay lists.
  
	  IDE Table					IDL Table
	-------------				      -------------
     e0: <s1, s1, NEG>				   dl0: [e0]
     e1: <s1, s1, A1>				   dl1: [e1]
     e2: <s4, s4, NEG>				   dl2: [e2]
     e3: <s3(Y), s3(Y), A31>			   dl3: [e4, e3]
     e4: <s2(X,Y), s2(X,Y), A21>		   dl4: [e2, e1, e4]
     e5: <s2(a,Y), s2(a,b), AaY>		   dl5: [e2, e1, e5]
     e6: <s2(X,Y), s2(a,Y), A22>		   dl6: [e2, e1, e6]
     e7: <s3(Y), s3(a), A32>			   dl7: [e6, e3]
						   dl8: [e6, e7]

  No simplification is possible and everything is undefined.
  ===========================================================================*/

:- table s1/0, s2/2, s3/1, s4/0.

s1 :- tnot(s1).				% creates e0, dl0.

s2(_,_) :- s1.				% creates e1, dl1.
s2(a,_) :- tnot(s4).

% In the following var(X) appears so that only subgoal s(VAR) gets answers

s3(X) :- var(X), tnot(s4).		% creates e6
s3(X) :- var(X), s1.
s3(X) :- var(X), s2(X,Y), s3(Y).	% creates e3, e4, dl3, dl7 and e7, dl8
					% (also answers A31, A32)
s3(X) :- tnot(s4), s1, s2(X,Y), Y = b.	% creates e5, dl4, dl5 and dl6

s4 :- tnot(s4).				% creates e2, dl2.

%----------------------------------------------------------------------------

test :- s1, fail.	% complete s1
test :- s2(X,_), s3(X), fail.
test.