Question 145
A circle with center $O$ and radius $R$ is drawn on a paper, and $A$ is a given point in the circle with $OA = a$. Fold the paper to make a point $A'$ on the circumference coincident with point $A$, then a crease line is left on the paper. Find out the set of all points on such crease lines, when $A'$ goes through every point on the circumference.