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