code: function(event, ui) {
            var content = ui.content;
            var filtered = _.filter(content, function(datum) {
              return !_.find(self.choices, { value: datum.value });
            });
            // "Why don't you just assign to ui.content?" jquery.ui.autocomplete
            // is holding a reference to the original array. If I assign to ui.content
            // I'm not changing that original array and jquery.ui.autocomplete ignores me.
            content.length = 0;
            $.each(filtered, function(i, datum) {
              content.push(datum);
            });
          }
docs: Stomp out suggestions of choices already made