function stringify(element, index, children, callback) {
  switch (element.type) {
    case IMPORT:
    case DECLARATION:
      return element.return = element.return || element.value;

    case COMMENT:
      return '';

    case KEYFRAMES:
      return element.return = element.value + '{' + serialize(element.children, callback) + '}';

    case RULESET:
      element.value = element.props.join(',');
  }

  return strlen(children = serialize(element.children, callback)) ? element.return = element.value + '{' + children + '}' : '';
}
@param {object} element
@param {number} index
@param {object[]} children
@param {function} callback
@return {string}