span>.replace tbl x y</code> replaces the current binding of <code class="code">x</code>
   in <code class="code">tbl</code> by a binding of <code class="code">x</code> to <code class="code">y</code>.  If <code class="code">x</code> is unbound in <code class="code">tbl</code>,
   a binding of <code class="code">x</code> to <code class="code">y</code> is added to <code class="code">tbl</code>.
   This is functionally equivalent to <a href="Hashtbl.html#VALremove"><code class="code"><span class="constructor">