Formula _ 263 denotes the set formula _ 264 . We write formula _ 265 for formula _ 266 , and formula _ 250 is called a pointed event model ( formula _ 247 often represents the actual event ) . Formula 263 represents the set formula 264 . We write formula 265 for formula 266 , and formula 250 is called a pointed event model ( formula 247 often denotes the actual event ) .