// So, when we thaw a MUT_ARR_PTRS_FROZEN, we must cope with two cases:
// either it is on a mut_list, or it isn't. We adopt the convention that
// So, when we thaw a MUT_ARR_PTRS_FROZEN, we must cope with two cases:
// either it is on a mut_list, or it isn't. We adopt the convention that