code: def get_b2aset(a2bset):
      
      b2aset = {}
      for a_item, bset in a2bset.items():
          for b_item in bset:
              if b_item in b2aset:
                  b2aset[b_item].add(a_item)
              else:
                  b2aset[b_item] = set([a_item])
      return b2aset
docs: Given gene2gos, return go2genes. Given go2genes, return gene2gos.