It's called a "race condition". I've seen it, but I try really hard to avoid it. Not always easy to do.

X uses Y, Y uses X. So let's go down the compiler's path for this. During the compilation of X, we see this statement "use Y;". So, we stop compiling X, load Y, and compile it. During that, we see the statement to "use X;". We stop and ... no, we see that X is already loaded. So we call X->import(), and continue. We finish compiling Y, including executing any of its global statements, and then go back to X and continue.

The solutions include:

• Breaking this relationship in one direction. Allow X to use Y, but not the other way around.
• Put all variables into functions. You'll need to watch out for infinite recursion if you leave your definitions as-is, I think. But I didn't fully grasp the code, so you may be ok.
• Put all common code into a common parent class. This is like the first option: breaking the relationship, except this does it in both directions. X uses W, and Y uses W, and W doesn't care about X nor Y.
• Put everything in the same compilation unit (module). :-)
• Others? I'm sure there are others. Regardless of what they are, they will usually be a redesign of some magnitude, large or small.