: build-lookup ( seq -- hashtable )
    H{ } [ set-of ] reduce-index ;

: build-pairs ( sum seq -- pairs )
    [ [ - ] keep 2array ] with map! ;

: lookup ( hashtable pairs -- index/f )
    first2 [ of ] bi-curry@ bi and ;

: find-sum ( hashtable seq -- sum )
    [ lookup ] with map! sift ;

: two-sum ( sum seq -- pair )
    [ nip build-lookup ] [ build-pairs ] 2bi find-sum ;