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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _format_alignment(self, a1, a2):
 """Returns `a1` marked up with HTML spans around characters that are also at the same index in `a2`. :param a1: text sequence from one witness :type a1: `str` :param a2: text sequence from another witness :type a2: `str` :rtype: `str` """

        html = []
        for index, char in enumerate(a1):
            output = self._substitutes.get(char, char)
            if a2[index] == char:
                html.append('<span class="match">{}</span>'.format(output))
            elif char != '-':
                html.append(output)
        return ''.join(html)