ts can processed correctly by using both the sum of the weights and the
sum of the weights-squared for each bin. The newly added option <tt>SumW2Error()</tt> implements a similar
strategy for (unbinned) weighted ML fits by applying a correction to the covariance matrix
as follows
<pre>
   V' = V C-1 V
</pre>
where <tt>V</tt> is the covariance matrix from the fit to weighted data, and <tt>C-1</tt> is the inverse of the
covariance matrix calculated from a similar likelihood that constructed with the event wei