sp;s</code> appends the string pattern <code class="code">s</code> at the end
   of buffer <code class="code">b</code> with substitution.
   The substitution process looks for variables into
   the pattern and substitutes each variable name by its value, as
   obtained by applying the mapping <code class="code">f</code> to the variable name. Inside the
   string pattern, a variable name immediately follows a non-escaped
   <code class="code">$</code> character and is one of the following:</p>
<ul>
<li>a non