docs: function (cfg) {
          var change = function () {
              if ($.trim ($(this).val ()) === '') { cfg.empty (true) }
              else                                { cfg.empty (false) } }
          return this
              .keyup (change)
              .change (change)
              .focus (_.bind (cfg.focus || _.noop, cfg, true))
              .blur (_.bind (cfg.focus || _.noop, cfg, false)) }