4. (10 marks) Find a most general unifier (if one exists) of the following pairs. - P(y,a,b,y) and P(c,f,g,f) where a and b are constants, and f,g,y are variables. - P(f(x),r(x),c) and P(w,r(q),q), where c is a constant, and x,w,q are variables