theorem
  g is commutative implies g "**" <* d1,d2 *> = g "**" <* d2,d1 *>
proof
  assume
A1: g is commutative;
  thus g "**" <* d1,d2 *> = g.(d1,d2) by Th12
    .= g.(d2,d1) by A1,BINOP_1:def 2
    .= g "**" <* d2,d1 *> by Th12;
end;
