'Checks kwargs for the values specified in \'requires\', which is a tuple
of strings. These strings are the NVP names of the required values.'

def _check_required(self, requires, **kwargs):

    for req in requires:
        if ((req.lower() not in kwargs) and (req.upper() not in kwargs)):
            raise PayPalError(('missing    required    :    %s' % req))
