anchor.diff:              M33d.Zero
           else
               let n = float x._count
  -            let sum = V3d(x._lhs.M03, x._lhs.M13, x._lhs.M23)
  +            let sum = x._sum
               let sumSq = V3d(x._lhs.M00, x._lhs.M11, x._lhs.M22)
               let off = V3d(x._lhs.M12, x._lhs.M02, x._lhs.M01)
   
  
positive.code: 
              let rhs = l._rhs + r._rhs
              let count = l._count + r._count
  
              SphereRegression3d(l._reference, lhs, rhs, count)
         
      static member (-) (l : SphereRegression3d, r : SphereRegression3d) =
          if l._count <= r._count then SphereRegression3d.Empty
  
negative.code:         if x._count < 2 then
              V3d.Zero
          else
              let sum = x._sum
              let off = V3d(x._lhs.M12, x._lhs.M02, x._lhs.M01)
              let n = float x._count
              let avg = sum / n
  