ARB_ROOTIDX
#define ARB_ROOT(head) ARB_NODE(head, ARB_ROOTIDX(head))
ARB_ROOTIDX(head) = _tmpidx; \
ARB_ROOTIDX(head) = _tmpidx; \
ARB_ROOTIDX(head) = ARB_SELFIDX(head, child); \
ARB_ROOTIDX(head) = ARB_SELFIDX(head, elm); \
ARB_ROOTIDX(head) = ARB_SELFIDX(head, child); \
ARB_ROOTIDX(head) = ARB_SELFIDX(head, elm); \