add optional angle radius parameter in curbeBezier `curveStep` curves about d3-shape HOT 4 OPEN

move to x1,y1
xa = (x1+x2)/2; // x1+r if stepBefore, x2-r if stepAfter
ya = (y1+y2)/2;
sx = Math.sign(x2-x1); 
sy = Math.sign(y2-y1); 
if (|x2-x1| > 2r && |y2-y1| > 2r):
  line to xa - sx * r, y1 
  arc to xa, y1 + sy * r
  line to xa, y2 - sy * r
  arc to xa + sx * r, y2
  line to x2, y2
else if (|x2-x1| > 2r)
  … // line + bezier + line?
else if (|y2-y1| > 2r)
  … // ?? smaller r?
else
  ... // bezier ??