addChild = (parent, child, sortMethod, prevProp = '_prev', nextProp = '_next') => {
  let prev = parent._tail;
  while (prev && sortMethod && sortMethod(prev, child)) prev = prev[prevProp];
  const next = prev ? prev[nextProp] : parent._head;
  prev ? prev[nextProp] = child : parent._head = child;
  next ? next[prevProp] = child : parent._tail = child;
  child[prevProp] = prev;
  child[nextProp] = next;
}
@param  {Object} parent
@param  {Object} child
@param  {Function} [sortMethod]
@param  {String} prevProp
@param  {String} nextProp
@return {void}