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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 last_string(self):
 """The last entry in code_builder, or ``None`` if none so far."""

        cb = self.code_builder
        len_cb = len(cb)
        if len_cb > 0:
            return cb[len_cb - 1]
        else:
            return None