> if not (null back_loops){- && not (f `elem` ls')-} then
> --if length back_loops > 1 then panic "barf!" else
> d2c (head back_loops) `thenUs` \core_e ->
> if not (null back_loops){- && not (f `elem` ls')-} then
> --if length back_loops > 1 then panic "barf!" else
> d2c (head back_loops) `thenUs` \core_e ->