USING: math.matrices sequences.deep memoize ;
MEMO: fibs ( n -- seq )
 1 + { { 1 1 } { 1 0 } } <array>
 2 identity-matrix [ m. ] binary-reduce ;
MEMO: fib ( n -- r )
 dup 3 mod [ - fibs flatten reverse
 { 0 1 3 } swap nths ] keep swap nth ;