In content model of elements, we may refer to model classes in alternation, sequenceOptional, sequenceOptionalRepeatable, seuanceRepeatable and sequence modes. Why not interleave ? Moreover, why not being able to add an element in a classe with a min cardinality and a max one, i.e. <classes><memberOf key="someClass" min="something" max="sometingElse"/>. Of course, this makes sense only for sequence, alternation and interleave.