generators.ET0LTreeGrammar:
( { t0, t1, t2, t3 },
  { r:0, l:0, u:0, d:0, .:2 },
  {
    {
      t0 -> (t0 . t1),
      t1 -> (t2 . t1),
      t2 -> (t2 . t3),
      t3 -> (t0 . t3)
    },
    {
      t0 -> u,
      t1 -> l,
      t2 -> d,
      t3 -> r
    }
  },
  t0
)
