gistfile1.clj

; you can just take the first two ?b's because they come into
; bufferop sorted

(defbufferop top-2 [tuples] 
  (take 2 tuples))

; let cascalog do the sorting (grouped by ?a) for you  

(??<- [?a ?b-out] 
      (data ?a ?b) 
      (:sort ?b)
      (:reverse true)
      (top-2 ?b :> ?b-out)) 

; and when you have more vars that you want to define the grouping with
; you just add them to the output vars
; this gives you the top-2 ?b for every ?a ?c ?d ?e

??<- [?a ?b-out ?c ?d ?e] 
      (data ?a ?b ?c ?d ?e) 
      (:sort ?b)
      (:reverse true)
      (top-2 ?b :> ?b-out))