function isEnd(pt1, pt2, pt3, pt4, pt5, pt6, { 
maxStroke = 10, angleTol = 20 } = {}) {
  const angleApprox = (a1, a2) => Math.abs(a1-a2) < angleTol;
  if (pt1.on || !pt2.on || pt3.on || pt4.on || !pt5.on || pt6.on) return false;
  if (dist(tuple(pt2), tuple(pt5)) > maxStroke * 1.4) return false;
  const angle1 = rad2deg(angle(tuple(pt5), tuple(pt2), tuple(pt1)));
  const angle2 = rad2deg(angle(tuple(pt6), tuple(pt5), tuple(pt2)));
  if (!angleApprox(angle1 + angle2, 160)) return false;
  return true;
}
Test if the points construct an end
@param { ControlPoint } pt1 
@param { ControlPoint } pt2 
@param { ControlPoint } pt3 
@param { ControlPoint } pt4 
@param { ControlPoint } pt5 
@param { ControlPoint } pt6 
@param { { maxStroke: number? } } config
@returns { boolean }