code: function(node, callback, depth) {
  			if (this.isSkipped(node)) {
  				return;
  			}
  
  			var childNode,
  				childNodes = node && node.childNodes || [];
  
  			depth = (depth || 1);
  
  			for (var n = 0, len = childNodes.length; n < len; n++) {
  				childNode = childNodes[n];
  
  				// callback can return false to stop recursive
  				if (callback(childNode, depth) !== false) {
  					this.walk(childNode, callback, depth + 1);
  				}
  			}
  		}
docs: call callback for each child of node