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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 string(self):
 r"""This is valid if and only if 1. the expression is a :class:`.TexCmd` AND 2. the command has only one argument. :rtype: Union[None,str] 'Hello' 'Hello World' \textbf{Hello World} """

        if isinstance(self.expr, TexCmd) and len(self.expr.args) == 1:
            return self.expr.args[0].value