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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 fill_arg(self, *args):
 """ If we get a single positional argument, and only a single variable needs to be filled, fill that variable with the value from that positional argument. """

        missing = self.missing_vars()
        if len(args) == len(missing) == 1:
            self.setval(missing[0], args[0])