: (find-pair) ( seq target -- pair )
    '[ first2 + _ = ] find nip ;

: find-pairs ( target seq -- seq pair/f )
   [ 2 <combinations> swap find-pair ] keep ;

: find-indices ( seq pair/f -- indices/f )
    [ '[ _ index ] map ] when* ;

: two-sum ( target seq -- pair )
     find-pairs find-indices ;