m._compute_oneletter() -- [utility] Set the oneletter member variable

def _compute_oneletter(self):
        """
        m._compute_oneletter() -- [utility] Set the oneletter member variable
        """
        letters = []
        for i in range(self.width):
            downcase = None
            if self.bits[i] < 0.25:
                letters.append('.')
                continue
            if self.bits[i] < 1.0: downcase = 'True'
            tups = [(self.ll[i][x],x) for x in ACGT if self.ll[i][x] > 0.0]
            if not tups:  #Kludge if all values are negative (can this really happen?)
                tups = [(self.ll[i][x],x) for x in ACGT]
                tups.sort()
                tups.reverse()
                tups = [tups[0]]
                downcase = 'True'
            tups.sort()      #Rank by LL
            tups.reverse()
            bases = [x[1] for x in tups[0:2]]
            bases.sort()
            if len(bases) == 2: L = two2one[''.join(bases)]
            else:               L = bases[0]
            if downcase: L = L.lower()
            letters.append(L)
        self.oneletter = ''.join(letters)