Visitor Pattern:

AbstractVisitor : ConcreteVisitor.
ConcreteVisitor = <d> Data.

A = B C.
B : X | Y.
X = .
Y = .

A {
  traverse(ConcreteVisitor v) {
    v.visit_A(this);
    b.traverse(v);
    c.traverse(v); } }
B {
  traverse(ConcreteVisitor v) { 
    v.visit_B(this); } }
X {
  traverse(ConcreteVisitor v) { 
    super.traverse(v);
    v.visit_X(this); }}

AbstractVisitor {
  void visit_A(A host) {}
  void visit_B(B host) {}
  void visit_X(X host) {}
}
ConcreteVisitor {
  void visit_A(A host) { ... }
  void visit_X(X host) { ... use d ... }
}