code: function getNthChildString(elm, selector) {
  	const siblings =
  		(elm.parentNode && Array.from(elm.parentNode.children || '')) || [];
  	const hasMatchingSiblings = siblings.find(
  		sibling => sibling !== elm && axe.utils.matchesSelector(sibling, selector)
  	);
  	if (hasMatchingSiblings) {
  		const nthChild = 1 + siblings.indexOf(elm);
  		return ':nth-child(' + nthChild + ')';
  	} else {
  		return '';
  	}
  }
docs: Given an element and a selector that finds that element (but possibly other sibling elements)
  return the :nth-child(n) pseudo selector that uniquely finds the node within its siblings
  @param {Element} elm			 The Element
  @param {String} selector	 The selector
  @return {String}					 The nth-child selector