fixed issues with definition of bf, and changed bf (which was df) to an actual bf.

Signed-off-by: Pepijn Kokke <pepijn.kokke@gmail.com>
defined the calkin-wilf tree, and made a start with wrestling with the stern-brocot tree.

Signed-off-by: Pepijn Kokke <pepijn.kokke@gmail.com>
proved N_pos

Signed-off-by: Pepijn Kokke <pepijn.kokke@gmail.com>