generators.ET0LTreeGrammar:
(
  { A, B, F },
  { F:0, conc:5, conc:2, enc+:1, enc+:2, enc-:1, enc-:2, 
    empty:0, -:1, +:1, enc:1 },
  {
    {
      A -> conc[enc[A],F,enc+[B],enc-[B],enc-[F,-[A]]],
      B -> conc[F,enc-[A],enc+[B,B],F,enc+[F,+[B]]],
      F -> conc[F,F]
    },
    {
      A -> empty,
      B -> empty,
      F -> F
    }
  },
  A, 1+ 2
)
