struct PK_bound_def_s
    {
    PK_bound_t        bound;        --- method of defining bound
    PK_LOGICAL_t      forward;      --- whether bound in positive sense of path
    double            distance;     --- extrusion distance
    PK_ENTITY_t       entity;       --- bounding entity
    PK_LOGICAL_t      nearest;      --- whether divisions are numbered from
                                    --- nearest to or furthest from the
                                    --- profile
    int               nth_division; --- select nth division
    };
typedef struct PK_bound_def_s PK_bound_def_t;



This structure describes how an extruded body or feature is bound in one
direction.


It has the following fields:

bound        method used for defining this bound.  See the documentation
               for PK_bound_t

forward      whether the bound is on the side of the given profile
               in the direction of the given path vector, or on the other
               side.

distance     distance to bound when bound is PK_bound_distance_c.
               This must be greater than or equal to zero.
               To extrude in the opposite direction to the path
               vector, forward is set to PK_LOGICAL_false.

entity       a bounding entity.  This may be a body, face or surface
               depending upon bound.

nearest      if PK_LOGICAL_true then divisions are numbered starting
               from one and increasing in the direction moving away from
               the profile.  If nearest is set to PK_LOGICAL_false then
               the first division is that furthest from the profile and
               division numbers increase towards the profile.

nth_division when bound is one of: PK_bound_surf_c, PK_bound_face_c,
               PK_bound_body_c or PK_bound_sheet_t then nth_division may be
               set to select a particular division.