generators.regularTreeGrammar:
  ( {A, B, C, D},
    {r:0, l:0, u:0, d:0, .:2},
    {
      A -> (A . B) weight 4,
      A -> u,
      B -> (C . B) weight 4,
      B -> l,
      C -> (C . D) weight 4,
      C -> d,
      D -> (A . D) weight 4,
      D -> r
    },
    A
  )
