common_prefix
common_prefix(bt, &lprefix, &uprefix, &mp->prefix);
common_prefix(bt, &srckey, &dst->prefix, &tmpkey);
common_prefix(bt, &src->prefix, &dst->prefix, &dstpfx);
static void common_prefix(struct btree *bt, struct btkey *min,
!common_prefix(s_out->subnet_addr, s_in->subnet_addr,