The answer is “it depends on how you defined the operation that acts on the circle”
As long as, C*Cinv = I, where C is the circle, Cinv is the inverse of circle, and I the identity. You're right. C, I, and * are entirely up in the air.
Basically: Given a function `f`, then the function `g` is the inverse of `f` if `g(f(x)) = x`.