struct PK_blend_setback_data_s
    {
    int                  n_edges;   --- number of edges (0)
    const PK_EDGE_t    * edges;     --- edge (NULL)
    const PK_LOGICAL_t * which_end; --- whether setback is on
                                    --- vertex at end of edge (NULL)
    const double       * distances; --- setback distance (NULL)
    };
typedef struct PK_blend_setback_data_s PK_blend_setback_data_t;




This structure records information specifying whether the
end of a blend on an edge will be setback.


This structure contains data which is used to define the shape
of the blend where three or more blends meet at a vertex. Specifying a
setback will result in the blend being trimmed back before it meets the blends
on neighbouring edges, creating a larger patch in the gap left.

edges        is an array of length n_edges specifying on which edges
               the blend shall be setback.

which_end    is an array of length n_edges specifying on which
               end of the corresponding edge the setback shall occur.
               If `which_end[i]' is set to PK_LOGICAL_true, the
               setback shall occur at the vertex at the end of the edge.
               If `which_end[i]' is set to PK_LOGICAL_false, the
               setback shall occur at the vertex at the start of the edge.

distances    is an array of n_edges non-negative doubles
               specifying the distance in which the blend
               should be setback on the corresponding edge.
               The blend shall be setback by approximately this distance
               measured from the place where the blend meets the blends
               on its two neighouring edges.