struct PK_FACE_neutral_causes_s
    {
    PK_FACE_t    neutral_face;   --- Face in neutral sheet.
    int          n_faces_back;   --- Number of faces compressed onto
                                 --- neutral_face from behind.
    PK_FACE_t   *faces_back;     --- Set of faces compressed onto
                                 --- neutral_face from behind.
    int          n_faces_front;  --- Number of faces compressed onto
                                 --- neutral_face from in front.
    PK_FACE_t   *faces_front;    --- Set of faces compressed onto
                                 --- neutral_face from in front.
    };
typedef struct PK_FACE_neutral_causes_s PK_FACE_neutral_causes_t;





This data structure contains the tracking information for a face in a
neutral sheet.




A face on a neutral sheet is usually the result of intersecting the
images of the projections of two faces from the corresponding face set
pair, one from the first set and one from the the second set.  There
are cases, however, in which the number of faces which project to the
neutral sheet is zero or two or more.  The faces listed in
faces_back are the faces from the second set which project onto
neutral_face and those listed in faces_front are the faces in the
first set which project onto neutral_face.