generators.ET0LTreeGrammar:
  ( { F },
    { shrink3:4, conc:2, 2+:2, +:2, 2-:1, 6-:1, F:0, enc:2 },
    {
      { F -> shrink3[F, 2+[F,+[F,6-[F]]], 2-[F], F] }
    },
    F
  )
