static void printUnrolledStringTree(StringTree1 tree) {
  printUnrolledStringTree("", tree);
}

static void printUnrolledStringTree(S prefix, StringTree1 tree) {
  if (tree == null) ret;
  if (tree.leaf) print(prefix);
  for (S s : keys(tree.children))
    printUnrolledStringTree(appendSpaceIfNempty(prefix) + s, tree.children.get(s));
}