static L mapLike mapCodeTokens(O f, L l) {
  new L out;
  for i over l: {
    O o = l.get(i);
    out.add(odd(i) ? callF(f, o) : o);
  }
  ret out;
}

static L mapCodeTokens(L l, O f) {
  ret mapCodeTokens(f, l);
}