A NarrowingTransformer that narrows cascadingly to the “less abstract” type of the visited subject by switching on the case types and invoking the proper transformX method.

For example:

// ValueExample has the case types Primary | Operation
shared actual default Result transformValueExpression(ValueExpression that) {
    switch (that)
    case (is Primary) { return transformPrimary(that); }
    case (is Operation) { return transformOperation(that); }
}

I honestly don’t know when you’d want to use this instead of ImmediateNarrowingTransformer; perhaps the strongest argument for its existence is that it was trivial to generate along with other generated code while ceylon.ast was originally written, but would be significantly more painful to create (by hand or automatically) afterwards.