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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 get_value_for_expr(self, expr, target):
 """I have no idea."""

        if expr in LOGICAL_OPERATORS.values():
            return None
        rvalue = expr['value']
        if rvalue == HISTORICAL:
            history = self.history[target]
            if len(history) < self.history_size:
                return None
            rvalue = sum(history) / float(len(history))

        rvalue = expr['mod'](rvalue)
        return rvalue