rank-two primitive: d((y_a+y_b-y_c) tensor e_abc) = 2 y_L tensor e_ab
symmetric average: normalization cancels the m-2 identical summands exactly
top diagonal: d3[G;abc] is the oriented boundary of a three-edge triangle
top diagonal: d2*d3 = 0 exactly in the pointed-hyperplane graph
line insertion: (r-3)+rank{a,b,c} = (r-3)+2 = r-1 < r
conclusion: every third-point insertion has zero projection to K_(3,r)(M;E)
