private static void checkThatExplicitIndexingChainInheritsCorrectly(ComponentRegistry<DocprocChain> allChains,
                                                                        ChainSpecification chainSpec) {
        ChainSpecification.Inheritance inheritance = chainSpec.inheritance;
        for (ComponentSpecification componentSpec : inheritance.chainSpecifications) {
            ChainSpecification parentSpec = getChainSpec(allChains, componentSpec);
            if (containsIndexingChain(allChains, parentSpec)) return;
        }
        throw new IllegalArgumentException("Docproc chain '" + chainSpec.componentId +
                                           "' must inherit from the 'indexing' chain");
    }
Returns indexing cluster implicitly created by this,
or empty if an explicit cluster is used (or if called before the build phase)
config-model/src/main/java/com/yahoo/vespa/model/content/Content.java