[R] how to evaluate a cubic Bezier curve (B-spline?) given the four control points

[Zack Weinberg]

I'm trying to use R to determine the quality of a cubic Bezier curve
approximation of an elliptical arc.  I know the four control points
and I want to compute (x,y) coordinates of many points on the curve.
I can't find anything in either the base distribution or CRAN that
does this; all the spline-related packages seem to be about *fitting*
piecewise Bezier curves to a data set.  Presumably, internally they
have the capability I need, but it doesn't seem to be exposed in a
straightforward way.  Help?