theorem Th95:
  X1 c= Y1 & X2 c= Y2 implies [:X1,X2:] c= [:Y1,Y2:]
proof
  assume X1 c= Y1 & X2 c= Y2;
  then [:X1,X2:] c= [:Y1,X2:] & [:Y1,X2:] c= [:Y1,Y2:] by Th94;
  hence thesis;
end;
