Return a string for each gene representing GO section membership.

def get_gene2aart(gene2section2gos, sec2chr):
        """Return a string for each gene representing GO section membership."""
        geneid2str = {}
        for geneid, section2gos_gene in gene2section2gos.items():
            letters = [abc if s in section2gos_gene else "." for s, abc in sec2chr.items()]
            geneid2str[geneid] = "".join(letters)
        return geneid2str