code: function appendAttrsKeysOnly(action, attrs, updates, opts) {
    if (typeof attrs === "string" || attrs instanceof String)
      attrs = attrs.split(",");
    
    if (attrs && isArray(attrs)) {
      for(var i = 0; i < attrs.length; i++) {
        var key = attrs[i];
        if (key && isStr(key)  && key.length > 0)
          appendAttr(action, key.trim(), null, updates, opts);
      }
    }
    return updates;
  }
docs: /*  Append attributes without a value.
  @param {String} action
  @param {String|Array} attrs key names sepcified as comma separated string or
  as an array of strings.
  @param {Updates} updates object.