private void validateUpgradePoliciesOfIncreasingConservativeness(List<Step> steps) {
        UpgradePolicy previous = Collections.min(List.of(UpgradePolicy.values()));
        for (Step step : steps) {
            UpgradePolicy strictest = previous;
            List<DeploymentInstanceSpec> specs = instances(List.of(step));
            for (DeploymentInstanceSpec spec : specs) {
                if (spec.upgradePolicy().compareTo(previous) < 0)
                    illegal("Instance '" + spec.name() + "' cannot have a looser upgrade " +
                            "policy than the previous of '" + previous + "'");

                strictest = Comparables.max(strictest, spec.upgradePolicy());
            }
            previous = strictest;
        }
    }
Throws an IllegalArgumentException if any instance has a looser upgrade policy than the previous
config-model-api/src/main/java/com/yahoo/config/application/api/DeploymentSpec.java