private Set<RegionName> prodRegions() {
        return steps().stream()
                      .flatMap(s -> s.zones().stream())
                      .filter(zone -> zone.environment().isProduction())
                      .flatMap(z -> z.region().stream())
                      .collect(Collectors.toSet());
     }
Returns the production regions the steps of this specifies a deployment to.
config-model-api/src/main/java/com/yahoo/config/application/api/DeploymentInstanceSpec.java