Holy shit. I spent two hours staring at how garbage this code was and didn't...

Holy shit. I spent two hours staring at how garbage this code was and didn't even realise it was #ifdef'd out behind a define not even mentioned in doomdef.h. It's not actually used anywhere (superseded entirely by the much nicer, much more relevant P_NewVertexSlope()... out with you, ancient, foul demons who should've been SPRINGCLEANed long ago.