static <A> A evalInQ(Q q, final F0<A> f) {
  if (isInQ(q)) ret callF(f);
  
  final new Var<Either<A, Throwable>> var;
  
  q.add(r {
    try {
      var.set(::<A, Throwable> eitherA(callF(f)));
    } catch e {
      var.set(::<A, Throwable> eitherB(e));
    }
  });
  
  ret returnOrThrow_either(waitForVarToBeNotNull(var));
}