public Set<Environment> concernedEnvironments() {
        return steps.stream().map(Step::zones)
                .flatMap(Collection::stream)
                .map(DeclaredZone::environment)
                .collect(Collectors.toSet());
    }
Returns the set of unique environments concerned by this deployment spec
config-model-api/src/main/java/com/yahoo/config/application/api/DeploymentSpec.java