e use of <code class="code">unsafe_to_string</code>, it is convenient to
    consider an "ownership" discipline. A piece of code that
    manipulates some data "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 ownershi