owever, for
    <code class="code">set_tag</code>, in the case of immutable blocks where the middle-end optimizers
    never see code that discriminates on their tag (for example records), the
    operation should be safe.  Such uses are nonetheless discouraged.</p>

<p>For experts only:
    <code class="code">set_field</code> et al can be made safe by first wrapping the block in
    <a href="Sys.html#VALopaque_identity"><code class="code"><span class="constructor">Sys</span>.opaque_identity</code></a>, so 