theorem
  G1 is_naturally_transformable_to G2 implies t(#)(id id B) = t
proof
  assume
A1: G1 is_naturally_transformable_to G2;
  then
A2: G1*id B is_naturally_transformable_to G2*id B by Th20;
  thus t(#)(id id B) = (t*id B)`*`id(G1*id B) by Th31
    .= t*id B by A2,NATTRA_1:24
    .= t by A1,Th32;
end;
