15.11.3  AVERAGE 

A  weighted  average  is  produced  of  the data.  The data is weighted by
1/DY**2 if DY is non zero.  Otherwise a weight of 1 is used.   The  result
is  X,Y,DY.   If the original DYs are all zero (or 1) then the final DY is
1/SQRT(points).  Where points is the number of points averaged to  form  a
bin.