lex step derivative</strong></span> can be used.
      The idea is very simple: Since <span class="emphasis"><em>f</em></span> is complex-differentiable,
      <span class="emphasis"><em>f(x+&#8520; h) = f(x) + &#8520; hf'(x) - h<sup>2</sup>f''(x) + &#119926;(h<sup>3</sup>)</em></span>.
      As long as <span class="emphasis"><em>f(x)</em></span> &#8712; &#8477;, then <span class="emphasis"><em>f'(x)
      = &#8465; f(x+&#8520; h)/h + &#119926;(h<sup>2</sup>)</em></span>. This method requires a single
     