theorem Th20:
  Fracmult(x,Fracmult(y,z)) = Fracmult(Fracmult(x,y),z)
  proof
    [x`1*(y`1*z`1),x`2*(y`2*z`2)]
    = [x`1*(y`1*z`1),(x`2*y`2)*z`2] by GROUP_1:def 3
    .= Fracmult(Fracmult(x,y),z) by GROUP_1:def 3;
    hence thesis;
  end;
