while ({
          val ap = argF.asInstanceOf[Ap[F, Any, Any]]
          argsF ::= ap.fp
          argsFLength += 1
          argF = ap.fn.asInstanceOf[FA[F, Any]]
          argF.isInstanceOf[Ap[F, _, _]]
        }) ()
        // consecutive `ap` calls have been queued as operations;
        // argF is no longer an `Ap` node, so the entire topmost left-associated
        // function application branch has been looped through and we've
        // moved (`argsFLength` - `lengthInitial`) arguments to the stack, through
        // (`argsFLength` - `lengthInitial`) `Ap` nodes, so the function on the right
        // which consumes them all must have (`argsFLength` - `lengthInitial`) arguments
        val argc = argsFLength - lengthInitial
        fns ::= Fn[G, Any, Any](foldArg(argF.asInstanceOf[FA[F, Any => Any]], f), argc)
        fnsLength += 1
        loop()