generators.ET0LTreeGrammar:
  ( {S, L, R, U, D},
    {r:0, l:0, u:0, d:0, empty:0, conc:7, conc:4, conc:2},
    {
      {
        S -> conc[S, R, S, L, U, S, D],
        R -> conc[R, R],
        L -> conc[L, L],
        U -> conc[U, U],
        D -> conc[D, D]
      },
      {
        S -> conc[r, l, u, d],
        R -> r,
        L -> l,
        U -> u,
        D -> d
      }
    },
    S
  )
