Consider this Prolog database:
	r(a,b).		 s(b,c).
	r(a,c).		 s(b,d).
	r(b,a).		 s(c,d).
	r(a,d).		 s(c,c).
	r(e,d).		 s(d,e).
	r(a).		 s(_).
Now consider this Prolog query, and mentally trace through this goal:
?-   r(X,Y), s(Y,Z), not(r(Y,X)), not(s(Y,Y))
(a) Without using a computer what is the first answer found to the query?
(b) How many times does the interpreter backtrack from the third to the second subgoal to get this first answer?
( c) What about these queries: 
		r(_ ,X), r(X).
		r(A,B), s(A),r(B).
		s(X,Y),s(Y),r(Y,Z),not(r(Z)).