def create_mutable_file(contents=None, keysize=None):
    """I create a new mutable file, and return a Deferred that will fire
        with the IMutableFileNode instance when it is ready. If contents= is
        provided (a bytestring), it will be used as the initial contents of
        the new file, otherwise the file will contain zero bytes. keysize= is
        for use by unit tests, to create mutable files that are smaller than
        usual."""
