struct PK_PARAM_sf_s
    {
    PK_INTERVAL_t       range;        --- permitted range of parameter values
    PK_PARAM_bound_t    bound[2];     --- token describing entity bounds
    PK_PARAM_periodic_t periodic;     --- token describing entity periodicity
    PK_PARAM_form_t     form;         --- token describing entity form
    PK_LOGICAL_t        continuous;   --- whether continuous
    };
typedef struct PK_PARAM_sf_s PK_PARAM_sf_t;



This standard form describes the parameterisation of a geometric entity in
one parametric direction.


Description of fields:

range:
   an interval defining the allowed range of parameter values.  If the
   parametric range is infinite, then the values given here will be
   sufficient to include all that part of the geometric entity within the
   size box.

bound:
   tokens describing the boundedness at either end of the parameter range.
   bound[0] describes the limit range.value[0], and bound[1] describes the
   limit range.value[1].  The token is one of the following:

      PK_PARAM_bound_infinite_c    entity extends infinitely
      PK_PARAM_bound_extendable_c  entity may be extended in this direction
      PK_PARAM_bound_bound_c       entity may not be extended
      PK_PARAM_bound_closed_c      ends of parameter range are coincident
      PK_PARAM_bound_degenerate_c  for surfaces only, parameter boundary has
                                   no 3D extent

periodic:
   token indicating whether the parameterisation is periodic.  It has one of
   the values:

      PK_PARAM_periodic_no_c       not periodic
      PK_PARAM_periodic_yes_c      periodic continuously differentiable at seam
      PK_PARAM_periodic_seamed_c   periodic but not continuously
                                   differentiable across seam

form:
   token indicating the relationship between the parameterisation and 3-space
   extent.  It takes one of the following values:

      PK_PARAM_form_linear_c       parameter is proportional to distance along
                                   a straight line.  For a surface the straight
                                   line corresponds to a constant value of the
                                   other parameter
      PK_PARAM_form_circular_c     parameter represents an angle round a circle
                                   For a surface, the circle corresponds to a
                                   constant value of the other parameter.
      PK_PARAM_form_any_c          not one of the above

continuous:
   if PK_LOGICAL_true then all derivatives are continuous, otherwise all
   derivatives are not necessarily continuous.