Control behavior for accumulators.
Is this object valid? This Lua object holds a reference to an object within the game engine. It is possible that the game-engine object is removed whilst a mod still holds the corresponding Lua object. If that happens, the object becomes invalid, i.e. this attribute will be
false. Mods are advised to check for object validity if any change to the game state might have occurred between the creation of the Lua object and its access.