I've implemented the following features for composite indexes:

• create
• find_exact
• remove

I've also added respective tests.

One question I have regarding the composite index is whether it should support a find_partial feature. For example, given the following structure:

{
  (thing1, thing2): [doc1, doc2],
  (thing1, thing3): [doc3, doc4]
}

should find((thing1, _)) return [doc1, doc2, doc3, doc4]?