def getAlphabet(self):
        s = "".join(map(lambda x: x.mString, self.values())).lower()
        s = re.sub("[%s]" % "".join(self.mGapChars), "", s)
        ss = re.sub("[acgtxn]", "", s)
        if float(len(ss)) < (len(s) * 0.1):
            return "na"
        else:
            return "aa"
get alphabet from the multiple alignment.
dataset/ETHPy150Open CGATOxford/cgat/CGAT/Mali.py/Mali.getAlphabet