PK_ERROR_code_t PK_CURVE_eval_handed
(
--- received arguments ---
PK_CURVE_t      curve,              --- curve
double          t,                  --- curve parameter
int             n_derivs,           --- number of derivatives
PK_HAND_t       hand_direction,     --- evaluation direction

--- returned arguments ---
PK_VECTOR_t     p[]                 --- point and derivatives.
)


The function behaves like PK_CURVE_eval but takes an additional argument to
control the direction of the evalution.


Specific Errors:
    PK_ERROR_bad_parameter            cannot evaluate out of range
    PK_ERROR_too_many_derivatives     too many derivatives requested
    PK_ERROR_eval_failure             internal evaluator failure


This function is the handed version of PK_CURVE_eval, which evaluates the point
 and derivatives at a given parameter of a curve.

The point and derivatives at a given parameter are determined using the
curve segment that the parameter lies in. If the parameter lies on a
segment boundary then there are two possible curve segments that can be used.
They are the segment before the boundary (hand_direction == PK_HAND_left_c )
and the segment after the boundary (hand_direction == PK_HAND_right_c).

        0            1            2          3          4          5
        |------------|------------|----------|----------|----------|
                                    on_left  |
                                   <---------| on_right
                                             |--------->
                                             t

Where derivative discontinuities exist at the parameter, the derivatives
calculated by the left segment may be different from those calculated
using the right segment.

If the curve is periodic, the left (or right) segment of the beginning of
the periodic range equals to the left (or right) segment of the end of the
periodic range.