def foldr(op,initial,seq):
    if not seq: #seq is empty
        return initial
    else:
        return op(seq[0], foldr(op, initial, seq[1:]))  #recursion on the rest of the list

def foldl(op, z, seq):
    if not seq:
        return z
    else:
        return foldl(op, op(z, seq[0]), seq[1:])