owns" it; there are several disjoint ownership
    modes, including:</p>
<ul>
<li>Unique ownership: the data may be accessed and mutated</li>
<li>Shared ownership: the data has several owners, that may only
      access it, not mutate it.</li>
</ul>
<p>Unique ownership is linear: passing the data to another piece of
    code means giving up ownership (we cannot write the
    data again). A unique owner may decide to make the data shared
    (giving up mutation rights on it), but shared data may not become
 