docs: def status(self):
          """Return string indicating the error encountered on failure."""
          self._check_valid()
          if self._ret_val == swiglpk.GLP_ENOPFS:
              return 'No primal feasible solution'
          elif self._ret_val == swiglpk.GLP_ENODFS:
              return 'No dual feasible solution'
          return str(swiglpk.glp_get_status(self._problem._p))