private static boolean singleInheritance(Collection<NewDocumentType> specInheriteds) {
        if (specInheriteds.isEmpty()) return true;
        if (specInheriteds.size()>1) return false;
        return singleInheritance(specificInheriteds(specInheriteds.iterator().next()));
    }
The Java class the class of the given type should inherit from. If the input type inherits from _one_
other type, use that, otherwise Document.
vespa-documentgen-plugin/src/main/java/com/yahoo/vespa/DocumentGenMojo.java