sub A {
  BARE_BLOCK: {
    if (0 == $_[0]) {
      return 1 + $_[1];
    }
    elsif (1 == $_[0]) {
      return 2 + $_[1];
    }
    elsif (2 == $_[0]) {
      return 3 + 2*$_[1];
    }

    @_=($_[0] - 1, $_[1] ? A($_[0], $_[1]-1) : 1);
    redo BARE_BLOCK;
  }
}