IN: ...
USING: math sequences kernel prettyprint ;

: matrix-nth ( n matrix -- elt )
    [ first length [ rem ] [ /i ] 2bi ] keep nth nth ;
    
: matrix-length ( matrix -- n )
    [ length ] [ first length * ] bi ;

! example 
{ { 3 4 } 
  { 2 1 }
  { 5 7 } 
  { 3 2 } } dup matrix-length [ swap matrix-nth . ] with each