PK_ERROR_code_t PK_CURVE_eval
(
--- received arguments ---
PK_CURVE_t      curve,      --- curve
double          t,          --- curve parameter
int             n_derivs,   --- number of derivatives

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


This function evaluates a point and derivatives at a given parameter on the
given curve.


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


The function calculates the Cartesian coordinates of the point at the given
curve parameter t, and also the derivatives with respect to t, up to
order n_derivs.

In the array of returned data:

            p[0] is the point itself
            p[1] is the first derivative
            p[2] is the second derivative
            p[n] is the nth derivative

It is not necessarily required that t lies inside the range as given by
PK_CURVE_ask_param.  If the curve is PK_PARAM_periodic_yes_c or
PK_PARAM_periodic_seamed_c then any value is acceptable. If t is outside
the range and the curve is PK_PARAM_bound_extendable_c, then the evaluation
will be performed on the extension.

Where derivative discontinuities exist at a junction between segments, the
derivatives just after the discontinuity are returned, i.e. on the right.
An exception to this is where the curve is periodic, and t is at the top
of the periodic range, in which case evaluation is on the left (parameters
which are equal to the top of the range, plus a non-zero multiple of the
period, still evaluate from the right).

The maximum number of derivatives that can be computed depends on the class
of the given curve:

    curve type:               maximum number of derivatives
    -----------               -----------------------------
    line/circle/ellipse       2

    intersection curve        2

    foreign geometry          determined by external evaluator

    constant parameter        determined by underlying surface evaluator
    line

    SP-curve                  2

    B-curve                   any number