generators.ET0LTreeGrammar:
( {AB, AC, BA, BC, CA, CB},
  {r:0, l:0, u:0, d:0, conc:2, conc:3},
  {
    {
      AB -> conc[AC,AB,CB],
      AC -> conc[AB,AC,BC],
      BA -> conc[BC,BA,CA],
      BC -> conc[BA,BC,AC],
      CA -> conc[CB,CA,BA],
      CB -> conc[CA,CB,AB]
    },
    {
      AB -> r,
      AC -> u,
      BA -> l,
      BC -> conc[l,u],
      CA -> d,
      CB -> conc[d,r]
    }
  },
  AB
)
