  In both cases, at least one of the integer and fractional parts must be
   given; the exponent part is optional.
   The <code class="code">_</code> (underscore) character can appear anywhere in the string
   and is ignored.
   Depending on the execution platforms, other representations of
   floating-point numbers can be accepted, but should not be relied upon.
   Raise <code class="code"><span class="constructor">Failure</span> <span class="string">"float_of_string"</span></code> if the given string is n