code: function _subclassObject(tree, base, extension, extName) {
  		// $.ui.fancytree.debug("_subclassObject", tree, base, extension, extName);
  		for (var attrName in extension) {
  			if (typeof extension[attrName] === "function") {
  				if (typeof tree[attrName] === "function") {
  					// override existing method
  					tree[attrName] = _makeVirtualFunction(
  						attrName,
  						tree,
  						base,
  						extension,
  						extName
  					);
  				} else if (attrName.charAt(0) === "_") {
  					// Create private methods in tree.ext.EXTENSION namespace
  					tree.ext[extName][attrName] = _makeVirtualFunction(
  						attrName,
  						tree,
  						base,
  						extension,
  						extName
  					);
  				} else {
  					$.error(
  						"Could not override tree." +
  							attrName +
  							". Use prefix '_' to create tree." +
  							extName +
  							"._" +
  							attrName
  					);
  				}
  			} else {
  				// Create member variables in tree.ext.EXTENSION namespace
  				if (attrName !== "options") {
  					tree.ext[extName][attrName] = extension[attrName];
  				}
  			}
  		}
  	}
docs: Subclass `base` by creating proxy functions