struct PK_blend_y_blend_data_s
    {
    PK_LOGICAL_t         y_blend;      --- whether to y blend
                                       --- (PK_LOGICAL_false)
    int                  n_vertices;   --- number of vertices (0)
    const PK_VERTEX_t   *vertices;     --- array of vertices to blend (NULL)
    };
typedef struct PK_blend_y_blend_data_s PK_blend_y_blend_data_t;




This structure contains information specifying whether or not y-shaped
vertex blends are required, and a list of vertices specifying where to apply
them.


When two blends on adjacent edges of opposite convexity meet at
a three, or more, edge vertex and the default shape of the blend
is to roll one blend over the other, a y-shaped vertex blend offers
an alternative shape of the blend at the vertex.

y_blend         when set to PK_LOGICAL_true switches y-shaped vertex
                  blending on.
                  The default is PK_LOGICAL_false.

vertices        is an array of length n_vertices specifying on
                  which vertices to apply y-shaped vertex blends.

If y-shaped vertex blending is switched on and no vertices are supplied then a
y-shaped vertex blend will be applied to all suitable vertices.

See Creating Y-shaped blends chapter in the Functional Description.