function log2(number)
*	       ====
*     determine 2_log of number
*     -------------------------
      if(number.le.0)
     /   stop 'log2: invalid argument'
      n=number
      do 10 k=1,64
         n=n/2
         log2=k-1
         if(n.eq.0) return
   10 continue
      stop 'log2: still looping'
      end