struct PK_SURF_trim_data_s
    {
    int                n_spcurves;  --- number of spcurves
    PK_SPCURVE_t      *spcurves;    --- spcurves
    PK_INTERVAL_t     *intervals;   --- spcurve intervals
    int               *trim_loop;   --- which trim loop
    int               *trim_set;    --- which trim set
    };
typedef struct PK_SURF_trim_data_s PK_SURF_trim_data_t;




This structure records surface trimming data


Trim data for a surface consists of zero or more trim loops making up zero
or more trim sets.  Each trim set represents a connected piece of trimmed
surface.  Each trim loop is made up of trimmed SP-curves.  Each trimmed
SP-curve is represented by an SP-curve and a parametric interval.

spcurves consists of an array of SP-curves which describe the boundary
           loops of the trimmed surface.  More information on spcurves is
           provided in the documentation of PK_FACE_output_surf_trimmed
           and PK_SURF_make_sheet_trimmed.


intervals is an array of each SP-curve's bounding interval.

Trim sets and trim loops are identified by integers.  When a
PK_SURF_trim_data_t is returned from PK_FACE_output_surf_trimmed, then trim
sets and trim loops will each be allocated identifiers commencing from 0 and
increasing by one.  When a PK_SURF_trim_data_t is input to
PK_SURF_make_sheet_trimmed, there are no limitations on trim loop
identifiers.

As an example, if there are two trim_sets and the first has 2 trim_loops, then
the fields may be as follows:

           spcurves            trim_loop            trim_set

              1                    0                   0
              2                    0                   0
              3                    0                   0
              4                    0                   0
              5                    1                   0
              6                    1                   0
              7                    1                   0
              8                    2                   1
              9                    2                   1
             10                    2                   1
             11                    2                   1

trim_set 0 has trim_loops 0 and 1. trim_loop 0 has spcurves 1 to 4 and
trim_loop 1 has spcurves 5 to 7.
trim_set 1 has trim_loop 2 which has spcurves 8 to 11.