generators.ET0LTreeGrammar:
( { L, R, U, "L'", "R'", "U'" },
  { h:2, v:2, c:1, a:0, b:0 },
  {
    {
      L -> h[L,v[a,U]],
      R -> h[v[a,U],R],
      U -> v[a,U]
    },
    {
      L -> h["L'",v[b,"U'"]],
      R -> h[v[b,"U'"],"R'"],
      U -> v[b,"U'"]
    },
    {
      "L'" -> h["L'",v[a,"U'"]],
      "R'" -> h[v[a,"U'"],"R'"],
      "U'" -> v[a,"U'"]
    },
    {
      "L'" -> a,
      "R'" -> a,
      "U'" -> a
    }
  },
  c[h[L,h[v[a,U],R]]], 1* 2 3* 4
)
