Event Card. An event card detects and signals the occurrence of a specific event (for example, a collision, etc).
The only message an event card needs to receive is Check, which should check to see if the event occurred. If so, it should return the event (a GEvent object). If not, it should return null or undefined.
||Called to check if the event happened. It should return a GEvent object if the event occurred, or undefined if it didn't. If this is a "predicate card" that is, a card that simply checks if a certain condition is true (like "there are fewer than 6 slimes in the world"), it can also return a boolean (true/false) to indicate its evaluation of that predicate.|