### Summary 
This paper describes a method for using a hybrid of impulse- and constraint-based methods for enforcing nonpenetration constraints
### Problem 
Constraint-based methods are good for resting contact, but ill-suited for collisions, wherease impulse-based methods are good for momentary/colliding contact, but perform poorly in long-term contact.
### Key Ideas 
This paper suggests that a good method for simulating arbitrary interactions of rigid bodies would be to combine impulse- and constraint-based methods.