Updates need to use mutable in a few ways that are not fully implemented yet.

Here's what we're pending on:

+ Support for lazy reading: portion of the underlying bson would be "parsed" and parts would be "opaque"

+ Support for copying serialized portions in bulk: generating a new bson objs out of a mutated one should require re-encoding the entire object. Portion of the original object may be memcpy-ed to the destination object.

+ Support in-placeness: mutable can be "locked" for only in place ops and would generate a "damage vector" with write intent information.

+ Support for algs: find is already in place, sort is needed (e.g. in $push)

+ General clean up, since quite a few notions are being introduced.