struct PK_BODY_create_sheet_planar_o_s
    {
    int                    o_t_version;    --- version number (1)
    const int             *loops;          --- loop numbers (NULL)
    PK_AXIS1_sf_t          plane;          --- plane (XY plane)
    PK_AXIS2_sf_t          basis_set;      --- local coordinate system
                                           --- (world coordinate)
    };
typedef struct PK_BODY_create_sheet_planar_o_s PK_BODY_create_sheet_planar_o_t;



This options structure provides variable form input to
PK_BODY_create_sheet_planar.


The boundaries of the planar sheet are defined by a given set of
loops.  If the loops field is NULL (the default) then all the vectors
lie on the same loop, this loop must be defined in a clockwise direction
(when viewed in the direction of the surface normal) so as to provide the
exterior boundary of the sheet.  Any body created must have exactly one loop
defined clockwise, all other loops being defined anticlockwise.

In the case where the loops field is specified, for each vector in vectors,
the corresponding entry in loops is the loop index for the loop to which the
vector belongs.  Thus the number of elements in each array must be equal.

If the loops field of the options structure is NULL (the default), then the
given position vectors form a single loop which is the exterior of the sheet
body.

plane indicates the plane onto which all the position vectors in vectors
are projected.

plane and vectors are both defined in the local coordinate system.

The basis_set positions and orientates the local coordinate system in the
world coordinate system.