Dict: given a GO return a set of letters representing it's section membership(s).

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