+void btr_splice_out_node(struct btr_node *btrn);
+void btr_pushdown(struct btr_node *btrn);
+void *btr_context(struct btr_node *btrn);
+int btr_merge(struct btr_node *btrn);
+void btr_merge_to(struct btr_node *btrn, size_t dest_size);
+bool btr_eof(struct btr_node *btrn);