theorem
  (id a)+(id b) = id(a+b)
proof
  Hom(b,a+b) <> {} by Th61;
  then
A1: in2(a,b)*(id b) = in2(a,b) by CAT_1:29;
  Hom(a,a+b) <> {} by Th61;
  then in1(a,b)*(id a) = in1(a,b) by CAT_1:29;
  hence thesis by A1,Th66;
end;
