code: def subst_src_suffixes(self, env):
          
          memo_key = id(env)
          try:
              memo_dict = self._memo['subst_src_suffixes']
          except KeyError:
              memo_dict = {}
              self._memo['subst_src_suffixes'] = memo_dict
          else:
              try:
                  return memo_dict[memo_key]
              except KeyError:
                  pass
          suffixes = [env.subst(x) for x in self.src_suffix]
          memo_dict[memo_key] = suffixes
          return suffixes
docs: The suffix list may contain construction variable expansions,
          so we have to evaluate the individual strings.  To avoid doing
          this over and over, we memoize the results for each construction
          environment.