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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 _check_required(self, requires, **kwargs):
 """ Checks kwargs for the values specified in 'requires', which is a tuple of strings. These strings are the NVP names of the required values. """

        for req in requires:
            # PayPal api is never mixed-case.
            if req.lower() not in kwargs and req.upper() not in kwargs:
                raise PayPalError('missing required : %s' % req)