What's the gist of the setup for how a complex RPG rule system gets coded up? Like in D&D or a Diablo-like, you have base stats, equipment or classes modify those stats, sometimes the modifiers depend upon the circumstances, and maybe the modifiers get modified. So clearly you don't want to have a Belt of +4 Strength just do wearer.strength += 4 since at some point you'll have to take off the belt or are in an anti-magic field, etc. But what's a good and/or standard design for encoding all that kind of logic?