The CSI construction conjecture from locally homogeneous and VSI spacetimes

Let a spacetime be a constant scalar invariant (CSI) spacetime, meaning that all scalar polynomial curvature invariants are constant. A VSI spacetime is one for which all scalar polynomial curvature invariants vanish. CSI construction conjecture. If a spacetime is CSI, then it can be constructed from locally homogeneous spaces and VSI spacetimes. The source gives this as a further proposed structural description of CSI spacetimes; no proof or precise construction procedure is supplied there.