code: def get_variants_to_controllers(
          graph: BELGraph,
          node: Protein,
          modifications: Optional[Set[str]] = None,
  ) -> Mapping[Protein, Set[Protein]]:
      
      rv = defaultdict(set)
      variants = variants_of(graph, node, modifications)
      for controller, variant, data in graph.in_edges(variants, data=True):
          if data[RELATION] in CAUSAL_RELATIONS:
              rv[variant].add(controller)
      return rv
docs: Get a mapping from variants of the given node to all of its upstream controllers.