e know the
      values of PRED and PREY at time 0, we can use these finite difference
      equations to estimate the values of PRED and PREY at time DT.  These values
      can be used to get estimates at time 2*DT, and so on.  To get from time
      T_START = 0 to time T_STOP = 5, we simply take STEP_NUM steps each of size
      <pre>
        DT = ( T_STOP - T_START ) / STEP_NUM
      </pre>
    </p>

    <p>
      Because finite differences are only an approximation to derivatives, this
      process on