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>&nbsp;<span class="string">"float_of_string"</span></code> if the given string is not a vali