generators.regularTreeGrammar("skylines"):
  ( {S, B},
    {r:0, l:0, u:0, d:0, conc:2, conc:3},
    {
      S -> conc[S,r,S] weight 2,
      S -> B,
      S -> r weight .1,
      B -> conc[u,B,d] weight 3,
      B -> conc[B,r],
      B -> conc[r,B],
      B -> r weight .5
    },
    S
  )
