docs: def is_feasible(self, solution):
          """Returns True if the given solution's derivatives may have potential
          valid, complete solutions.
          """
          newvars = solution.new.keys()
          for newvar in newvars:
              for c in self._constraints_for_var.get(newvar, []):
                  values = c.extract_values(solution, use_defaults=True)
                  if not c(*values):
                      return False
          return True