public static CollisionRule GetGroupCollisionRuleDefault(CollisionRules a, CollisionRules b)
{
if (a.group == null || b.group == null)
return CollisionRule.Defer; //This can happen occasionally when objects aren't in a space or are being handled uniquely (like in compound bodies).
CollisionRule pairRule;
CollisionGroupRules.TryGetValue(new CollisionGroupPair(a.group, b.group), out pairRule);
return pairRule;
}
///<summary>