theorem
  F1 is_transformable_to F2 implies (idt id B)(#)p = p
proof
  assume
A1: F1 is_transformable_to F2;
  then
A2: (id B)*F1 is_transformable_to (id B)*F2 by Th10;
  thus (idt id B)(#)p = (idt (id B*F2)) `*` (id B*p) by Th18
    .= id B*p by A2,FUNCTOR2:5
    .= p by A1,Th20;
end;
