function findArcAngle(curve, relCircle) {
  const x1 = -relCircle.center[0];
  const y1 = -relCircle.center[1];
  const x2 = curve[4] - relCircle.center[0];
  const y2 = curve[5] - relCircle.center[1];

  return Math.acos(
    (x1 * x2 + y1 * y2) / Math.sqrt((x1 * x1 + y1 * y1) * (x2 * x2 + y2 * y2)),
  );
}
Finds angle of a curve fitting the given arc.

@param {ReadonlyArray<number>} curve
@param {Circle} relCircle
@returns {number}