Post a constraint to the colgen instance ColgenInstance which will be associated with the identifier Id. The constraint Cstr must be a valid colgen constraint of type >=/2,=:=/2,==/2,$=/2,$=. If Id is uninstantiated it will be unified with the external solver row number of the constraint when this is set up. The identifier can later be used to retrieve the dual value or subproblem cost function term associated with the constraint.