struct PK_BODY_loft_vector_clamp_s
    {
    int                n_tangents;  --- number of derivative vectors (0)
    const PK_VECTOR_t *tangents;    --- array of derivative vectors (NULL)
    int                n_vertices;  --- number of vertices to be clamped (0)
    const PK_VERTEX_t *vertices;    --- array of vertices to be clamped (NULL)
    };

typedef struct PK_BODY_loft_vector_clamp_s PK_BODY_loft_vector_clamp_t;



This structure holds an array of derivative vectors and an optional array
of corresponding vertices where the derivative vectors are to be applied.


The vector clamp structure has the following fields :

n_tangents        The number of derivatives held in the structure. This must
                    equal the value of n_vertices, or be 1.

tangents          An array of derivatives one for each vertex of vertices,
                    or a single vector "constant". If vertices is set to
                    NULL, they are then taken in order to correspond with
                    the vertices ordered around the profile commencing with
                    the start vertex. Note that vertices may not be set to
                    NULL in the case of a guide wire.

                    The start vertex will be understood to be the first vertex
                    on an open profile, and the specified start vertex on a
                    closed profile.

n_vertices        The number of vertices where the loft derivative is given.

vertices          An optional array of vertices where the loft derivative
                    is specified as the corresponding entry in tangents.
                    Setting vertices to NULL has the same effect as setting
                    it to be the collection of all vertices of the profile.

                    When specifying a vector clamp on a profile, if there are
                    guide wires specified for the loft, the loft derivative
                    along any guide wire is completely determined. Thus, those
                    vertices of the profile that have matches on guide wires
                    must not be included as entries of vertices. Due to the
                    effect of setting it to NULL, vertices must be explicitly
                    assigned to exclude profile vertices that have matches on
                    guide wires.

                    Similarly, when specifying a vector clamp on a guide wire,
                    the derivative along any profile is completely determined.
                    Thus, those vertices of the guide that have matches on
                    profiles must not be included as entries of vertices, and
                    vertices may not be set to NULL.