code> applies <code class="code">f</code> to all bindings in table <code class="code">tbl</code>.
   <code class="code">f</code> receives the key as first argument, and the associated value
   as second argument. Each binding is presented exactly once to <code class="code">f</code>.
<p>

   The order in which the bindings are passed to <code class="code">f</code> is unspecified.
   However, if the table contains several bindings for the same key,
   they are passed to <code class="code">f</code> in reverse 