sclass OccTree<A> {

  int count;

  new HashMap<A, OccTree> followUp;

  OccTree<A> parent;

  void add(L<A> script) {

    OccTree node = this;

    ++count;

    for (A c : script)

      (node = node.getOrMakeFollowUp(c)).count++;

  }

  OccTree getOrMakeFollowUp(A c) {

    OccTree node = followUp.get(c);

    if (node == null)

      followUp.put(c, node = newChild());

    ret node;

  }

  OccTree followUp(A a) {

    ret followUp.get(a);

  }

  Set<A> followUpKeys() {

    ret keys(followUp);

  }

  A randomChoice() {

    MultiSet<A> ms = new MultiSet(false);

    for (A c : keys(followUp))

      ms.add(c, followUp.get(c).count);

    ret msOneOf(ms);

  }

  OccTree<A> newChild() {

    new OccTree<A> child;

    child.parent = this;

    ret child;

  }

  // size including all nodes

  int size() {

    int n = 1;

    for (OccTree t : values(followUp))

      n += t.size();

    ret n;

  }

  L<OccTree<A>> allLeaves() {

    new L l;

    collectLeaves(l);

    ret l;

  }

  void collectLeaves(L<OccTree> l) {

    if (isLeaf())

      l.add(this);

    else

      for (OccTree t : values(followUp))

        t.collectLeaves(l);

  }

  bool isLeaf() {

    ret isEmpty(followUp);

  }

}