docs: def lookup(self, pathogenName, sampleName):
          """
          Look up a pathogen name, sample name combination and get its
          FASTA/FASTQ file name and unique read count.
  
          This method should be used instead of C{add} in situations where
          you want an exception to be raised if a pathogen/sample combination has
          not already been passed to C{add}.
  
          @param pathogenName: A C{str} pathogen name.
          @param sampleName: A C{str} sample name.
          @raise KeyError: If the pathogen name or sample name have not been
              seen, either individually or in combination.
          @return: A (C{str}, C{int}) tuple retrieved from self._readsFilenames
          """
          pathogenIndex = self._pathogens[pathogenName]
          sampleIndex = self._samples[sampleName]
          return self._readsFilenames[(pathogenIndex, sampleIndex)]