theorem Th38:
  for o being Object of A st F1, F2 are_naturally_equivalent holds
  e"!o = (e!o)"
proof
  let o be Object of A;
  assume
A1: F1, F2 are_naturally_equivalent;
  then F2 is_transformable_to F1;
  hence e"!o = e".o by FUNCTOR2:def 4
    .= (e!o)" by A1,Def6;
end;
