PFOV (Permissive Field of View) clean-room reimplemented based on the algorithm described in http://roguebasin.roguelikedevelopment.org/index.php?title=Precise_Permissive_Field_of_View, though the general structure is more influenced by recursive shadow casting, as implemented in Shadow.hs. In the result, this algorithm is much faster than the original algorithm on dense maps, since it does not scan areas blocked by shadows.