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  /**     @brief The max size of a key we can write, or 0 for computed max.
    *
    *      This macro should normally be left alone or set to 0.
    *      Note that a database with big keys or dupsort data cannot be
    *      reliably modified by a liblmdb which uses a smaller max.
    *      The default is 511 for backwards compat, or 0 when #MDB_DEVEL.
    *
    *      Other values are allowed, for backwards compat.  However:
    *      A value bigger than the computed max can break if you do not
    *      know what you are doing, and liblmdb <= 0.9.10 can break when
    *      modifying a DB with keys/dupsort data bigger than its max.
    *
    *      Data items in an #MDB_DUPSORT database are also limited to
    *      this size, since they're actually keys of a sub-DB.  Keys and
    *      #MDB_DUPSORT data items must fit on a node in a regular page.
    */
  // #ifndef MDB_MAXKEYSIZE
  // #define MDB_MAXKEYSIZE   ((MDB_DEVEL) ? 0 : 511)
  // #endif
  /**     The maximum size of a key we can write to the environment. */
  #if MDB_MAXKEYSIZE
  #define ENV_MAXKEY(env) (MDB_MAXKEYSIZE)
  #else
  #define ENV_MAXKEY(env) ((env)->me_maxkey)
  #endif