generators.ET0LTreeGrammar("rounded-off corners"):
( { S0, S, Sl, Sr },
  { F:9, S0:0, S:0, Sl:0, Sr:0 },
  {
    {
      S0 -> F[Sl,S,S,S,S,S,S,S,Sr],
      S -> F[S,S,S,S,S,S,S,S,S],
      Sl -> F[Sl,S,S,S,S,S,S,S,S],
      Sr -> F[S,S,S,S,S,S,S,S,Sr]
    }
  },
  S0
)
