e a positive non-zero value less than or equal to
      <code>max_denominator</code>.
    </p>
    <p>
      If a maximum denominator is not specified, or is <code>null</code> or
      undefined, the denominator will be the lowest value necessary to represent
      the number exactly.
    </p>
    <p>
      See <a href='#Errors'>Errors</a> for the treatment of other non-integer or
      out of range <code>max_denominator</code> values.
    </p>
    <pre>
x = new BigNumber(1.75)
x.toFraction()            // 