There's a lot of classes in rx-scheduler.hpp like recursed_scope_type, recursed_scope, recursed, recursion, etc.

Is there some general principle tying these together?

Calling what(state->r.get_recurse()); seems to be the general pattern to invoke these, but even after reading the code in rx-scheduler.hpp it's hard to understand what the line of code is actually supposed to do?

Based off history like #2 I am guessing the intent was to eliminate virtual calls.

However it's not obvious how it's doing this, and why simply exhausting the entire queue in a single loop wasn't done instead?

   while (!q.empty()) { 
     auto what = q.pop();
     what();
   }