GNUmifluz: SH_CIRCLEQ_INSERT_BEFORE

do { \
 (elm)->field.scqe_next = SH_PTR_TO_OFF(elm, listelm); \
 (elm)->field.scqe_prev = (elm)->field.scqe_next - \
 SH_CIRCLEQ_PREVP(listelm, field, type)->field.scqe_next;\
 if (SH_CIRCLEQ_PREVP(listelm, field, type) == (void *)(head)) \
 (head)->scqh_first = SH_PTR_TO_OFF(head, elm); \
 else \
 SH_CIRCLEQ_PREVP(listelm, \
 field, type)->field.scqe_next = \
 SH_PTR_TO_OFF(SH_CIRCLEQ_PREVP(listelm, \
 field, type), elm); \
 (listelm)->field.scqe_prev = -(elm)->field.scqe_next; \
} while (0)