I would like to add more functions to XMLNode.

I tried this way:
XMLList.prototype.function::someFunction= function someFunction() {
return this.toString() + "someFunction";
};


but yql parser returns error:
Error: missing name after . operator<javascript>:8:XMLList.prototype.function:^:someFunction= function someFunction() {
Error: syntax error<javascript>:10:}^;
Exception: Compilation produced 2 syntax errors. (<javascript>#1)

How can I implement XMLNode prototypes?