: prep-hashtable ( sum seq -- hashtable sum seq )
    tuck [ H{ } [ spin [ set-at ] keep ] reduce-index ] 2dip ;

: complements ( sum seq -- seq' )
    [ - ] with map ;

: find-indices ( key assoc -- index/f index/f )
    [ [ ?of nip ] with find ] keepd at ;

: result/empty ( index/f index/f -- pair )
    2dup and [ 2array ] [ 2drop { } ] if ;

: two-sum ( sum seq -- pair ) 
    prep-hashtable complements find-indices result/empty ;