theorem Th49:
  a is terminal implies for h being Morphism of a,a holds id a = h
proof
  assume a is terminal;
  then consider f being Morphism of a,a such that
A1: for g being Morphism of a,a holds f = g;
  let h be Morphism of a,a;
  id a = f by A1;
  hence thesis by A1;
end;
