theorem Th12:
  for f1,f2 being Morphism of a,[1]C holds f1 = f2
proof
  let f1,f2 be Morphism of a,[1]C;
  [1]C is terminal by Def8;
  then consider f being Morphism of a,[1]C such that
A1: for g being Morphism of a,[1]C holds f = g;
  thus f1 = f by A1
    .= f2 by A1;
end;
