such-that[op] seq filters the given sequence to those elements that return |yes> or |true> for op applied to that element it is less powerful than filter() but on the other hand, it should be faster
is-hungry |Fred> => |no> is-hungry |Sam> => |yes> is-hungry |Emma> => |yes> is-hungry |Liz> => |no> such-that[is-hungry] rel-kets[*] |Sam> + |Emma> such-that[is-prime] srange(|1>, |20>) |2> . |3> . |5> . |7> . |11> . |13> . |17> . |19>