theorem
  Product<*a1,a2,a3*> = a1 * a2 * a3
proof
  thus Product<*a1,a2,a3*> = Product(<*a1,a2*>^<*a3*>) by FINSEQ_1:43
    .= Product<*a1,a2*> * a3 by GROUP_4:6
    .= a1 * a2 * a3 by FINSOP_1:12;
end;
