struct PK_SWEPT_sf_s
    {
    PK_CURVE_t            curve;         --- curve to sweep
    PK_VECTOR1_t          direction;     --- vector direction to sweep
    };

typedef struct PK_SWEPT_sf_s PK_SWEPT_sf_t;




Specific Errors:
    PK_ERROR_unsuitable_entity      curve is not of allowed class


This standard form represents a swept surface, that is, the surface obtained by
sweeping the given curve in the given direction.

The curve must be of one of the following classes:

    PK_CLASS_line
    PK_CLASS_circle
    PK_CLASS_ellipse
    PK_CLASS_bcurve

The surface is parameterised as:

    C = curve
    D = direction

    S(u,v) = C(u) + vD

The range for u is the parameter range of curve, and the range of v is
infinite in each direction.