applications.collages.gridAlgebra:
( 4,
  {
    F0 = 5bc6cd9ab10da,
    F = 1ab-2ab-3ab-5ab-0ab-4bc-6da-7ab-8ab-9da-11bc-15ab,
    Fi1 = 10ab-12ab-13ab-14ab-0ab-4bc-6da-7ab-8ab-9da-11bc-15ab-3ab,
    Fi = 10ab-12ab-13ab-14ab-0ab-4bc-6da-7ab-8ab-9da-11bc-15ab,
    S0 += { },
    S1 += { filledPolygon((-20,-20),(0,-20),(20,0),(20,20))[.5,.5,.5] },
    S += { filledPolygon((-20,-20),(20,-20),(20,20))[.5,.5,.5] },
    Si1 += { filledPolygon((-20,-20),(20,-20),(20,20))[1,1,1],
             filledPolygon((0,-20),(20,-20),(20,0))[.5,.5,.5] },
    Si += { filledPolygon((-20,-20),(20,-20),(20,20))[1,1,1] }
  }
)
