docs: def is_newer_b(a, bfiles):
      """
      check that all b files have been modified more recently than a
      """
      if isinstance(bfiles, basestring):
          bfiles = [bfiles]
  
      if not op.exists(a): return False
      if not all(op.exists(b) for b in bfiles): return False
  
      atime = os.stat(a).st_mtime # modification time
      for b in bfiles:
          # a has been modified since
          if atime > os.stat(b).st_mtime:
              return False
      return True