At the moment I implement `DirectRecover1` from https://www.cs.purdue.edu/homes/apothen/Papers/hessian-coloring-joc.pdf, but `DirectRecover2` is more efficient
At the moment I implement
DirectRecover1from https://www.cs.purdue.edu/homes/apothen/Papers/hessian-coloring-joc.pdf, butDirectRecover2is more efficient