 - see <a class="link" href="../ellint/ellint_2.html" title="Elliptic Integrals of the Second Kind - Legendre Form">ellint_2</a>.
      </p>
<p>
        Let's suppose we know the arc length and one radii, we can then calculate
        the other radius by inverting the formula above. We'll begin by encoding
        the above formula into a functor that our root-finding algorithms can call.
      </p>
<p>
        Note that while not completely obvious from the formula above, the function
        is completely