docs: def _get_x(self, kwargs):
          '''
          Returns x if it is explicitly defined in kwargs.
          Otherwise, raises TypeError.
          '''
          if 'x' in kwargs:
              return round(float(kwargs['x']), 6)
          elif self._element_x in kwargs:
              return round(float(kwargs[self._element_x]), 6)
          elif self._type == 3 and self._element_1mx in kwargs:
              return round(1. - float(kwargs[self._element_1mx]), 6)
          else:
              raise TypeError()