generators.ET0LTreeGrammar:
( { S, L, R },
  { r:0, l:0, u:0, d:0, conc:4, conc:5, conc:6, conc:9 },
  {
    {
      S -> conc[l, L, r, u, S, d, r, R, l],
      L -> conc[u, d, l, L, r],
      R -> conc[u, d, r, R, l]
    },
    {
      S -> conc[l,r,u,d,r,l],
      L -> conc[u,d,l,r],
      R -> conc[u,d,r,l]
    }
  },
  S
)
