code: def _get_batch_gender(items):
      
      genders = set([population.get_gender(x) for x in items])
      if len(genders) == 1:
          gender = genders.pop()
          if gender != "unknown":
              return gender
docs: Retrieve gender for a batch of items if consistent.
  
      Better not to specify for mixed populations, CNVkit will work
      it out
      https://github.com/bcbio/bcbio-nextgen/commit/1a0e217c8a4d3cee10fa890fb3cfd4db5034281d#r26279752