code: function (id, version, done, headers, config) {
          if (!_.isNumber(parseInt(id, 10))) {
            return done(new Error('id must be specified.'));
          }
          if (!_.isEmpty(version) && !_.isNumber(parseInt(version, 10))) {
            return done(new Error('version must be a number.'));
          }
  
          this._request(['files', id, 'versions', version], 'DELETE', done, null, null, null, headers, null, config);
        }
docs: Discards a specific file version to the trash.
  @summary Delete an Old Version of a File.
  @see {@link https://developers.box.com/docs/#files-delete-version}
  @param {number} id - The file's ID.
  @param {number} version - File version to promote.
  @param {requestCallback} done - The callback to invoke (with possible errors) when the request returns.
  @param {?RequestHeaders} [headers] - Additional headers.
  @param {?RequestConfig} [config] - Configure the request behaviour.