Method equal_range

src/include/cpp-btree/btree.h:1072–1074

  // the pair is equal to upper_bound(key).
  template <typename K>
  std::pair<iterator, iterator> equal_range(const K &key) {
    return {lower_bound(key), upper_bound(key)};
  }
  template <typename K>
  std::pair<const_iterator, const_iterator> equal_range(const K &key) const {
    return {lower_bound(key), upper_bound(key)};