Method init_child

src/include/cpp-btree/btree.h:531–534

    c->set_position(i);
  }
  void init_child(int i, btree_node *c) {
    set_child(i, c);
    c->set_parent(this);
  }
  // Returns the position of the first value whose key is not less than k.
  template <typename K>
  SearchResult<int, is_key_compare_to::value> lower_bound(