generators.ET0LTreeGrammar:
( {D, A, B, C},
  {r:0, l:0, u:0, d:0, empty:0, conc:7},
  {
    {
      A -> conc[B,u,A,r,A,d,C],
      B -> conc[A,r,B,u,B,l,D],
      C -> conc[D,l,C,d,C,r,A],
      D -> conc[C,d,D,l,D,u,B]
    },
    {
      A -> empty,
      B -> empty,
      C -> empty,
      D -> empty
    }
  },
  A
)
