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

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