code: function( node, tagNames, onlyOfType ){
  			var i = node.children.length,
  				countAll = 0,
  				countOfType = 0,
  				child,
  				out = null;
  
  			while(i--) {
  				child = node.children[i];
  				if(child.nodeType === 1) {
  					if(tagNames){
  						// count just only-of-type
  						if(this.hasTagName(child, tagNames)){
  							out = child;
  							countOfType++;
  						}
  					}else{
  						// count all elements
  						out = child;
  						countAll++;
  					}
  				}
  			}
  			if(onlyOfType === true){
  				return (countOfType === 1)? out : null;
  			}else{
  				return (countAll === 1)? out : null;
  			}
  		}
docs: get child node limited by presence of siblings - either CSS :only-of-type or :only-child
  
  @param  {DOM Node} rootNode
  @param  {Array} tagNames
  @return {DOM Node || null}