code: def _check_input_files(nspc, parser):
          
          if not len(nspc.filenames) == 3:
              parser.print_help()
              msg = """
        3 Expected files; Expected content: study population association",
        {} Actual   files: {}""".format(len(nspc.filenames), ' '.join(nspc.filenames))
              raise Exception(msg)
          for fin in nspc.filenames:
              if not os.path.exists(fin):
                  return "*{}* does not exist".format(fin)
  
          return False
docs: check filename args. otherwise if one of the 3 filenames is bad
          it's hard to tell which one