docs: def summarizePosition(self, index):
          """
          Compute residue counts at a specific sequence index.
  
          @param index: an C{int} index into the sequence.
          @return: A C{dict} with the count of too-short (excluded) sequences,
              and a Counter instance giving the residue counts.
          """
          countAtPosition = Counter()
          excludedCount = 0
  
          for read in self:
              try:
                  countAtPosition[read.sequence[index]] += 1
              except IndexError:
                  excludedCount += 1
  
          return {
              'excludedCount': excludedCount,
              'countAtPosition': countAtPosition
          }