function toggleCheatSheet() {
        scope.helpVisible = !scope.helpVisible;

        // Bind to esc to remove the cheat sheet.  Ideally, this would be done
        // as a directive in the template, but that would create a nasty
        // circular dependency issue that I don't feel like sorting out.
        if (scope.helpVisible) {
          previousEsc = _get('esc');
          _del('esc');

          // Here's an odd way to do this: we're going to use the original
          // description of the hotkey on the cheat sheet so that it shows up.
          // without it, no entry for esc will ever show up (#22)
          _add('esc', previousEsc.description, toggleCheatSheet, null, ['INPUT', 'SELECT', 'TEXTAREA']);
        } else {
          _del('esc');

          // restore the previously bound ESC key
          if (previousEsc !== false) {
            _add(previousEsc);
          }
        }
      }
Toggles the help menu element's visiblity