docs: def _add_minimization_vars(self):
          """Add variables and constraints for L1 norm minimization."""
  
          self._z = self._prob.namespace(self._model.reactions, lower=0)
  
          # Define constraints
          v = self._v.set(self._model.reactions)
          z = self._z.set(self._model.reactions)
  
          self._prob.add_linear_constraints(z >= v, v >= -z)