TUPLE: cross first rest ;
C: <cross> cross ( seq cross -- cross )
: >cross< ( cross -- first rest )  dup first>> swap rest>> ;
INSTANCE: cross sequence
: indexes ( n cross -- first-n rest-n )  rest>> length /mod ;
: 2nth ( n1 n2 seq1 seq2 -- elt1 elt2 )  swapd [ nth ] 2bi@ ;
M: cross nth  [ indexes ] keep >cross< 2nth swap prefix ;
M: cross length  dup first>> length swap rest>> length * ;
: n<cross> ( seqs -- cross )
    [ { { } } clone ] [ unclip swap n<cross> <cross> ] if-empty ;