Modifying a node in a Scala AST

Question

I understand that it is possible to obtain a Tree corresponding to a Scala expression, and also to traverse the resulting tree. Suppose that a given tree node matches some criterion, what method calls do I need to be able to modify it in place?, i.e. replace it with some other Tree?

object traverser extends Traverser {
    var applies = List[Apply]()

    override def traverse(tree: Tree): Unit = tree match {
        case someCriterion => modifyNodeSomehow // How to do this?
        // Otherwise...
        case app @ Apply(fun, args) =>
            applies = app :: applies
            super.traverse(fun)
            super.traverseTrees(args)
        case _ => super.traverse(tree)
    }
}

Answer 1

In another question I was suggested to use Transformer to do something similar. AFAIK it is not possible to modify the tree in place, however.