PK_ERROR_code_t    PK_CURVE_eval_with_tangent
(
--- 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.
PK_VECTOR_t *const tangent     --- curve tangent
)


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


Specific Errors:
    PK_ERROR_bad_parameter            cannot evaluate out of range
    PK_ERROR_at_singularity           failed to evaluate tangent at singularity
    PK_ERROR_too_many_derivatives     too many derivatives requested
    PK_ERROR_eval_failure             internal evaluator failure


This function provides an extension to the functionality of PK_CURVE_eval;
all the documentation for that function also applies here.

The tangent returned is normalised, and will always be in the direction of
increasing parameter value.