code: def add_linear_constraints(self, *relations):
          
          constraints = []
  
          for relation in relations:
              if self._check_relation(relation):
                  constraints.append(Constraint(self, None))
              else:
                  for name in self._add_constraints(relation):
                      constraints.append(Constraint(self, name))
  
          return constraints
docs: Add constraints to the problem
  
          Each constraint is represented by a Relation, and the
          expression in that relation can be a set expression.