generators.regularTreeGrammar:
  ( {T, L, R},
    {r:0, l:0, u:0, d:0, conc:5, empty:0},
    {
      T -> u weight .1,
      T -> conc[u,L,u,R,T],
      L -> empty weight .5,
      L -> conc[l,u,L,d,r],
      R -> empty weight .5,
      R -> conc[r,u,R,d,l]
    },
    T
  )
