How will Prolog interpret this?

Question

How will prolog interpret this expression?

a(R) :- b(R,S), b(S,R).

Does this mean that a(R) will only return true if both b(R,S) and b(S,R) are true?

Answer 1

a(R) will only return true if both b(R,S) and b(S,R) are true?

Yes. The comma is a logical and. The rule:

a(R) :- b(R,S), b(S,R)

can thus be explained as:

"a(R) holds given there exists an S such that b(R, S) holds, and b(S, R) holds".

If for example you have a dataset like:

b(a, c).
b(a, d).
b(c, a).
b(b, c).

Then it will succeed for a(a), since b(a, c) holds and b(c, a). But it will not succeed for a(b), since b(b, c) holds, but not b(c, b).