///<summary>
 /// Adds a contact to be managed by the constraint.
 ///</summary>
 ///<param name="contact">Contact to add.</param>
 public override void AddContact(Contact contact)
 {
     contact.Validate();
     var penetrationConstraint = penetrationConstraintPool.Pop();
     penetrationConstraint.Setup(this, contact);
     penetrationConstraints.Add(penetrationConstraint);
     if (!twistFriction.isActive)
     {
         //This is the first real contact.  All constraints need to become active.
         twistFriction.Setup(this);
         slidingFriction.Setup(this);
     }
 }
        //TODO: PROBLEM IS that the add contact/remove contact, when they go from 0 -> !0 or !0 -> 0, the whole constraint is added/removed from the solver.
        //The Added/Removed contact methods here will run ambiguously before or after they are removed from the solver.
        //That ambiguous order doesn't really matter though, since everything that these add/remove methods do is local to this solver object and its children.
        //It doesn't go out and modify any external values on referenced entities.  That only happens when it's added or removed from the solver by whatever owns this object!

        //To avoid ANY ambiguity, some third party is now responsible for adding and removing contacts from this.

        ///<summary>
        /// Adds a contact to be managed by the constraint.
        ///</summary>
        ///<param name="contact">Contact to add.</param>
        public override void AddContact(Contact contact)
        {
            contact.Validate();
            var penetrationConstraint = penetrationConstraintPool.Pop();
            penetrationConstraint.Setup(this, contact);
            penetrationConstraints.Add(penetrationConstraint);

            var frictionConstraint = frictionConstraintPool.Pop();
            frictionConstraint.Setup(this, penetrationConstraint);
            frictionConstraints.Add(frictionConstraint);

        }
        protected virtual void OnContactAdded(Contact contact)
        {
            contact.Validate();
            //Children manage the addition of the contact to the constraint, if any.
            if (!suppressEvents)
            {
                CollidableA.EventTriggerer.OnContactCreated(CollidableB, this, contact);
                CollidableB.EventTriggerer.OnContactCreated(CollidableA, this, contact);
            }
            if (Parent != null)
                Parent.OnContactAdded(contact);

        }