 With regard to the latter, there are of course cases in which the decoherence functional vanishes just by numerical coincidence. But there are also systematic cases of vanishing of interference even without environmental monitoring, namely in the presence of ‘conservation-induced’ decoherence (see e.g. Halliwell 2010). As an example, take an isolated system (say, with discrete energy levels), and consider histories composed of projections onto its energy states at arbitrary times. Because energy is conserved, in the energy basis each individual component is following the Schrödinger equation without interfering with the other components, and the corresponding histories decohere. While some authors in the decoherent histories literature take conservation-induced decoherence to be a significant novelty of the theory, it should be noted that it lacks the robustness of environment-induced decoherence, since it lacks a mechanism that actively suppresses interference.