passed
          both x and y, and can return either p or q where these are related by:
        </p>
<p>
          <span class="inlinemediaobject"><img src="../../../../equations/ibeta_inv5.png"></span>
        </p>
<p>
          so at any point we can swap a for b, x for y and p for q if this results
          in a more favourable position. Generally such swaps are performed so that
          we always compute a value less than 0.9: when required this can then be
          subtracted from 1 without undue c