Vector and Matrix extend Writable, and while that was merely poorly coupled before, it will be an actual problem now that we have a separate submodule for matrix: this module should not depend on hadoop at all, ideally. Distributed matrix work, as well as simply Writable wrappers can go somewhere else (where? core? yet another submodule which depends on matrix?), but it would be really nice if we could produce an artifact which doesn't require Hadoop which has our core linear primitives.