sub fib (UInt $n) {
    (state @seen = 0,1,1)[$n] //= fib($n-1) + fib($n-2);
}

##</code><code>##

sub fib (UInt $n, UInt $curr = 0, UInt $next = 1) {
    return $curr unless $n;
    return fib($n-1, $next, $curr + $next);
}

##</code><code>##

multi fib (0)       { 0 }
multi fib (1)       { 1 }
multi fib (UInt $n) { fib($n-1) + fib($n-2); }