public Collection<DocumentType> getInheritedTypes() {
        return Collections.unmodifiableCollection(inherits);
    }
Returns the DocumentNames which are directly inherited by this
as a read-only collection.
If this document type does not explicitly inherit anything, the list will
contain the root type 'Document'

@return a read-only list iterator containing the name Strings of the directly
        inherited document types of this
document/src/main/java/com/yahoo/document/DocumentType.java