theorem Th13:
  K is commutative implies
  (a1*a2*a3) |^n = (a1 |^n)*(a2 |^n)*(a3 |^n)
  proof
    assume
A1: K is commutative;
    hence (a1*a2*a3) |^n = ((a1*a2) |^n)*(a3 |^n) by BINOM:9
    .= (a1 |^n)*(a2 |^n)*(a3 |^n) by A1,BINOM:9;
  end;
