GNUmifluz: SH_LIST_INSERT_HEAD

do { \
 if ((head)->slh_first != -1) { \
 (elm)->field.sle_next = \
 (head)->slh_first - SH_PTR_TO_OFF(head, elm); \
 SH_LIST_FIRSTP(head, type)->field.sle_prev = \
 SH_LIST_NEXT_TO_PREV(elm, field); \
 } else \
 (elm)->field.sle_next = -1; \
 (head)->slh_first = SH_PTR_TO_OFF(head, elm); \
 (elm)->field.sle_prev = SH_PTR_TO_OFF(elm, &(head)->slh_first); \
} while (0)