code: def input_files(self):
          
          ret_list = []
          for key, val in self.file_dict.items():
              # For input files we only want files that were marked as input
              if val & FileFlags.input_mask:
                  ret_list.append(key)
          return ret_list
docs: Return a list of the input files needed by this link.
  
          For `Link` sub-classes this will return the union
          of all the input files of each internal `Link`.
  
          That is to say this will include files produced by one
          `Link` in a `Chain` and used as input to another `Link` in the `Chain`