PROLOG: Please answer a and b

(a) try to trace through search process for Q2;

(b) try to trace through Q1, but with sub-goals reversed.

Who teaches what.

f1: teaches(john, database, s1).

f2: teaches(john, ai, s1).

f3: teaches(mary, os, s1).

f4: teaches(mary, ai, s2).

f5: teaches(paul, compiler, s1).

Query 1: Whats the course taught by John and Mary?

?- teaches(john, X, _), teaches(mary, X, _).

Query 2: Who teaches both os and compiler?

?- teaches(X, os, _), teaches(X, compiler, _).