theorem
  G1 is_naturally_transformable_to G2 implies t*F = t(#)id F
proof
  assume G1 is_naturally_transformable_to G2;
  then G1*F is_naturally_transformable_to G2*F by Th20;
  hence t*F = (t*F)`*`id(G1*F) by NATTRA_1:24
    .= t(#)id F by Th31;
end;
