When solving Sudoku on paper, I sometimes get to the point where I need to resort to guessing or bifurcation. At that point, I might guess one number, and continue solving, changing that guess if I arrive at a contradiction.
I've experimented with a couple of approaches, e.g., circling the guess and writing the consequences more lightly or small in a particular corner of each cell. But none of this works too well, particularly not if I need a second level of guessing.
Is anyone aware of a reliable notation in this situation? Do I need coloured pencils or transparencies?