(defun&nbsp;log2&nbsp;(x)<br />
&nbsp;&nbsp;&quot;Use&nbsp;the&nbsp;fact&nbsp;that&nbsp;log2&nbsp;x&nbsp;=&nbsp;n&nbsp;+&nbsp;log2&nbsp;(x&nbsp;/&nbsp;2^n)&nbsp;and&nbsp;choose&nbsp;n&nbsp;so&nbsp;that&nbsp;there&nbsp;are&nbsp;preserved&nbsp;as&nbsp;many&nbsp;significant&nbsp;digits&nbsp;as&nbsp;needed&nbsp;in&nbsp;x.&nbsp;In&nbsp;this&nbsp;case&nbsp;100&nbsp;digits,&nbsp;but&nbsp;it&nbsp;could&nbsp;be&nbsp;fewer&nbsp;according&nbsp;to&nbsp;the&nbsp;floating-point&nbsp;format&nbsp;used.&quot;<br />
&nbsp;&nbsp;(let&nbsp;((n&nbsp;(-&nbsp;(integer-length&nbsp;x)&nbsp;100)))<br />
&nbsp;&nbsp;&nbsp;&nbsp;(+&nbsp;n&nbsp;(log&nbsp;(float&nbsp;(ash&nbsp;x&nbsp;(-&nbsp;n))&nbsp;1d0)<br />
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;2))))<br />