Read GO file. Store results in: section2goids sections_seen. Return goids_fin.

def _read_txt(self, fin_txt, get_goids_only, exclude_ungrouped):
        """Read GO file. Store results in: section2goids sections_seen. Return goids_fin."""
        goids_sec = []
        with open(fin_txt) as istrm:
            # Lines starting with a GO ID will have that GO ID read and stored.
            #   * Lines that do not start with a GO ID will be ignored.
            #   * Text after the 10 characters in a GO ID will be ignored.
            section_name = None
            for line in istrm:
                if line[:3] == "GO:":
                    goids_sec.append(line[:10])
                elif not get_goids_only and ":" in line:
                    mtch = self.srch_section.match(line)
                    if mtch:
                        secstr = mtch.group(1)
                        if section_name is not None and goids_sec:
                            self.section2goids[section_name] = goids_sec
                        if not exclude_ungrouped or secstr != HdrgosSections.secdflt:
                            section_name = secstr
                            self.sections_seen.append(section_name)
                        else:
                            section_name = None
                        goids_sec = []
            if section_name is not None and goids_sec:
                self.section2goids[section_name] = goids_sec
        return goids_sec