What is `circle_event.lower_x`?

I'm trying to understand the algorithm (btw if there is a formal description -- will be glad to read it) and want to understand what is `circle_event.lower_x` field, I understand that `circle_event` class represents circle built during sweeping the plane and [from docstring](https://github.com/boostorg/polygon/blob/8a51f6f8fd2d87c852e95cfd202d38deb4000ce0/include/boost/polygon/detail/voronoi_structures.hpp#L231) I can see that
> `lower_x_` - leftmost `x`-coordinate;

For example, if we have a circle
does this point has coordinates `(lower_x, center_y)` or am I missing something?

If `lower_x` is the lefmost should it be always to the left of the `center_x`?