y sequence of alphanumeric or <code class="code">_</code> characters,</li>
<li>an arbitrary sequence of characters enclosed by a pair of
   matching parentheses or curly brackets.
   An escaped <code class="code">$</code> character is a <code class="code">$</code> that immediately follows a backslash
   character; it then stands for a plain <code class="code">$</code>.
   Raise <code class="code"><span class="constructor">Not_found</span></code> if the closing character of a parenthesized variable
   cannot