1: Call t(a,b) is true (OK)
2: Call t(b,b) is true (OK)
3. Call t(X,Y) succeeds: t(a,b)
3. Call t(X,Y) succeeds: t(a,c)
3. Call t(X,Y) succeeds: t(b,b)
3. Call t(X,Y) succeeds: t(b,c)
4. Call p(X,Y) succeeds: p(a,b)
4. Call p(X,Y) succeeds: p(a,c)
5. Call q(X,Y) succeeds: q(b,b)
5. Call q(X,Y) succeeds: q(b,c)
6. Call t(X,Y) succeeds: t(a,b)
6. Call t(X,Y) succeeds: t(a,c)
6. Call t(X,Y) succeeds: t(b,b)
6. Call t(X,Y) succeeds: t(b,c)
