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

theorem Th11:
   for g1,g2 being morphism of the CategoryStr of C
   st f1 = g1 & f2 = g2 & f1 |> f2 holds f1 (*) f2 = g1 (*) g2
   proof
     let g1,g2 be morphism of the CategoryStr of C;
     assume
A1:  f1 = g1 & f2 = g2;
     assume
A2:  f1 |> f2;
     then
A3:  g1 |> g2 by A1;
     thus f1 (*) f2 = (the composition of C).(f1,f2) by A2,Def3
     .= g1 (*) g2 by A1,A3,Def3;
   end;
