code: def get_go2chrs(sec2gos, sec2chr):
          
          go2chrs = {}
          for goid, sections in get_b2aset(sec2gos).items():
              go2chrs[goid] = set(sec2chr[s] for s in sections)
          return go2chrs
docs: Dict: given a GO return a set of letters representing it's section membership(s).