generators.regularTreeGrammar:
( { S, A, B },
  { F:4, G:4, H:4, sq:0, -:0 },
  {
    S -> F[A,A,A,A],
    A -> F[A,A,A,A],
    A -> F[B,B,B,B],
    B -> G[G[-,-,-,sq],G[-,-,sq,sq],G[-,sq,-,sq],S],
    B -> G[G[-,-,-,sq],G[-,-,sq,sq],G[-,sq,-,sq],sq] weight .5
  },
  S
)
