DropGd(Tab,Drp) 

 A gradebook is returned with all dropped grades set to NA.

 Tab  is a gradebook table as obtained from GBget


 Drp  is a list coding which grade types can be dropped; the names
      of each element are taken from the gradebook column names
      with out any numbers.  For example

             Drp=list(H=2,L=1)

      would indicate that two homeworks and one lab can be
      dropped.

      If you would like to prevent specific grades from being
      dropped you can supply a list consisting of an integer and a
      logical vector, as in 

             Drp=list(H=list(2,c(T,T,T,T,T,T,T,F,F)))

      This would used if you had 9 homeworks, you were going to let
      them drop two but they were not allowed to drop the last two.
      The logical list must have an element for each grade of that
      type. 
      
 One caution:  If you use the feature which prevents certain grades
               from being dropped it will be possible that a
               dropped grade avereage is *lower* than a non-dropped
               grade average.  This function makes no account of
               this; you should work out grades both way and then
               take the max.

 $Id: GdBkUtil.S,v 1.5 2000/02/02 21:31:53 nicholst Exp $