USE CASE: on scan only; want to have a "summing combiner" that rolls
up by (rowId, colfam, colqual) on all row keys where the client has
visibility.
below is a simple example that expresses the use case.
accumulo table holding student to professor relationship by departments
++++++
 rowId  colfam  colqual  colvis  val 
++++++
 student1  TAKES_CLASS_WITH  prof1  MATH_DEPT  1 
 student1  TAKES_CLASS_WITH  prof1  MATH_DEPT  1 
 student1  TAKES_CLASS_WITH  prof1  COM_SCI_DEPT  1 
 student1  TAKES_CLASS_WITH  prof1  COM_SCI_DEPT  1 
 student2  TAKES_CLASS_WITH  prof1  MATH_DEPT  1 
 student2  TAKES_CLASS_WITH  prof1  COM_SCI_DEPT  1 
++++++
with the summing combiner the results would be
++++++
 rowId  colfam  colqual  colvis  val 
++++++
 student1  TAKES_CLASS_WITH  prof1  MATH_DEPT  2 
 student1  TAKES_CLASS_WITH  prof1  COM_SCI_DEPT  2 
 student2  TAKES_CLASS_WITH  prof1  MATH_DEPT  1 
 student2  TAKES_CLASS_WITH  prof1  COM_SCI_DEPT  1 
++++++
 the math department can only see math department totals
 the com sci department can only see the com sci department total
 the office of the dean has both access
therefore when scanning (it wouldnt work for compaction), how
can you sum over colvis?
assuming you had both colvis access the desired results would be:
+++++
 rowId  colfam  colqual  val 
++++
 student1  TAKES_CLASS_WITH  prof1  4 
 student2  TAKES_CLASS_WITH  prof1  2 
+++++
