code: def retrieve_from_cache(self):
          
          if self.nocache:
              return None
          if not self.is_derived():
              return None
          return self.get_build_env().get_CacheDir().retrieve(self)
docs: Try to retrieve the node's content from a cache
  
          This method is called from multiple threads in a parallel build,
          so only do thread safe stuff here. Do thread unsafe stuff in
          built().
  
          Returns true if the node was successfully retrieved.