reserve C for CategoryStr;
reserve f,f1,f2,f3 for morphism of C;
reserve g1,g2 for morphism of C opp;

theorem Th24:
  for C being non empty with_identities CategoryStr, f being morphism of C
  st f is identity holds f |> f
  proof
    let C be non empty with_identities CategoryStr;
    let f be morphism of C;
    assume f is identity;
    then f is Object of C by Th22;
    hence thesis by Th23;
  end;
