generators.ET0LTreeGrammar:
  ( {branch},
    {F:4, G:4, branch:0, side:1, bot:1, top:1, -:0 },
    {
      {
        branch -> F[side[branch],side[branch],top[branch],bot[branch]],
        branch -> F[side[branch],side[branch],-,bot[branch]],
        branch -> G[side[branch],side[branch],top[branch],bot[branch]],
        branch -> G[side[branch],side[branch],-,bot[branch]]
      }
    },
    branch
  )
