code: def _cull(potential, matches, verbose=0):
      
      for match in matches: # don't yield duplicates
          if _samefile(potential[0], match[0]):
              if verbose:
                  sys.stderr.write("duplicate: %s (%s)\n" % potential)
              return None
  
      if not stat.S_ISREG(os.stat(potential[0]).st_mode):
          if verbose:
              sys.stderr.write("not a regular file: %s (%s)\n" % potential)
      elif not os.access(potential[0], os.X_OK):
          if verbose:
              sys.stderr.write("no executable access: %s (%s)\n" % potential)
      else:
          matches.append(potential)
          return potential
  
      return None
docs: Cull inappropriate matches. Possible reasons:
          - a duplicate of a previous match
          - not a disk file
          - not executable (non-Windows)
      If 'potential' is approved it is returned and added to 'matches'.
      Otherwise, None is returned.