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    sub fib (Int $n) {
        return 1 if $n < 2;
        state @seen[$n] //= fib($n-1) + fib($n-2);
    }

    sub fib (Int $n) {
        $n < 2 or state @seen[$n] //= fib($n-1) + fib($n-2);
    }

    my&fib:={$_<2or state@s[$_]//=fib($_-1)+fib $_-2}