Why not? Happens all the time. My love and I require each other in any load order; that's just fine.

What matters in mutual including is the state of the including module - it is not yet compiled in its entirety. If the included module relies on data structures of the including module which will be set up at run time in there, it won't get those. So, each module must be set up proper to be used by the other one, then use the other one.

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