applications.collages.gridAlgebra:
( 3,
  { F1 = 0ab-2bc-3bc-4ab-5da-7ab,
    F2 = 0ab-1cd-2bc-3bc-4ab-5ab-6ab-7dc-8bc,
    F3 = 0ab-1cd-2bc-3bc-5ab-6ab-7dc-8bc,
    D = 0ab-4ab-8ab,
    S += { polyline((-20,-20),(0,0),(20,-20)) },
    D += { polyline((-20,-20),(20,20)) }
  }
)
