T(PG(3,2)): n=15, rank=3, flats=52, Sylvester=True, third-points-per-pair=[1,1]
  degree 3: dim K2=1575, rank(d2)=510, rank(d3)=455, H2=610, block-sum=610, nonzero-blocks=36
  degree 4: dim K2=3675, rank(d2)=15, rank(d3)=3451, H2=209, block-sum=209, nonzero-blocks=1
  degree 5: dim K2=105, rank(d2)=0, rank(d3)=105, H2=0, block-sum=0, nonzero-blocks=0
degree 4 conclusion: the unique nonzero block is the rank-3 spanning block
