This is similar to calling function painter:curveto with the following arguments 

   painter:curveto(x0+x1,y0+y1,x0+x2,y0+y2,x0+x3,y0+y3)
where (x0,y0) are the coordinates of the previous current point.

The behavior of this function is undefined when the current point is not set.

See also: PostScript operator rcurveto.