code: function expandClassNames(node, options) {
  	const classNames = node.classList.reduce((out, cl) => {
  		// remove all modifiers and element prefixes from class name to get a base element name
  		const ix = cl.indexOf('_');
  		if (ix > 0 && !cl.startsWith('-')) {
  			out.add(cl.slice(0, ix));
  		    out.add(cl.slice(ix));
  			return out;
  		}
  
  		return out.add(cl);
  	}, new Set());
  
  	if (classNames.size) {
  		node.setAttribute('class', Array.from(classNames).join(' '));
  	}
  }
docs: Expands existing class names in BEM notation in given `node`.
  For example, if node contains `b__el_mod` class name, this method ensures
  that element contains `b__el` class as well
  @param  {Node} node
  @param  {Object} options
  @return {Set}