begin 2.2.0-SNAPSHOT. fix a problem with mutating code in pullUpdate (e.g. BiGroup) which might be executed multiple times when there are several reactions. Now the pull phase is cached just as the push phase. Calling `pullUpdate` must be avoided now (and thus the visibility changed), instead of `event.pullUpdate(pull)`, there is now `pull(event)` which is also much nicer to read. Furthermore, there are extractors now for expr tuple1 and tuple2 which should automatically kick in in any implementation of Type.