theorem
  g is commutative implies g "**" <* d1,d2,d3 *> = g "**" <* d2,d1,d3 *>
proof
  assume
A1: g is commutative;
  thus g "**" <* d1,d2,d3 *> = g.(g.(d1,d2),d3) by Th14
    .= g.(g.(d2,d1),d3) by A1,BINOP_1:def 2
    .= g "**" <* d2,d1,d3 *> by Th14;
end;
