Removes duplicated nucleotides at the start of REF and ALT.
        But always leaves at least one nucleotide in each of REF and ALT.
        eg if variant is at position 42, REF=GCTGA, ALT=GCA, then
        sets position=41, REF=CTGA, ALT=CA.
        Assumes only one ALT, and does nothing if there is >1 ALT

def remove_useless_start_nucleotides(self):
        '''Removes duplicated nucleotides at the start of REF and ALT.
        But always leaves at least one nucleotide in each of REF and ALT.
        eg if variant is at position 42, REF=GCTGA, ALT=GCA, then
        sets position=41, REF=CTGA, ALT=CA.
        Assumes only one ALT, and does nothing if there is >1 ALT'''
        if len(self.REF) == 1 or len(self.ALT) != 1:
            return

        i = 0
        while i < len(self.REF) and i < len(self.ALT[0]) and self.REF[i] == self.ALT[0][i]:
            i += 1

        if i > 0:
            self.REF = self.REF[i - 1:]
            self.ALT = [self.ALT[0][i - 1:]]
            self.POS += i - 1