code: function (modes) {
          var prefix = '';
          var user_prefixes = this.get('user_prefixes');
  
          if (typeof modes[0] !== 'undefined') {
              prefix = _.detect(user_prefixes, function (prefix) {
                  return prefix.mode === modes[0];
              });
  
              prefix = (prefix) ? prefix.symbol : '';
          }
  
          return prefix;
      }
docs: Figure out a valid prefix given modes.
  If a user is an op but also has voice, the prefix
  should be the op as it is more important.