If the grouping field is 1b, you get a distinct/unique set of records. See an example:
o)t:((+:)`a`b`c!(1 2 2 3;3 4 4 5;6 5 5 9)); o)0N#.(?[t;(); 1b; ()]) a b c ----- 1 3 6 2 4 5 3 5 9 o)
