MODEL REGION OPERATORS:

void GR_Faces(PGeomRegn gr, int *nf, PGeomFace *faces);

Get the model faces that form the boundary of a model region.

int GR_FaceDir(PGeomRegn gr, PGeomFace gf);

Get the sense in which a model region is using a face, i.e., is the
face normal pointing into the region (return value 0), out of the
region (return value 1), both (return value 2) or none (return value
-1). ***** MUST VERIFY THiS WITH CODE VALUES ******

int GR_OnBoundary(PGeomRegn this, PGeomEntity gent);