function a($)
    dim $b, $d
    $b = 0^$
    while $
        $a = (($b-($^) & 1)+1) * right($,1)
        $d = $d + $a-9*($a>9)
        $a = ($d mod 10 =) * ($>2&$<7&$b+($<4)=16) * $
        $ = left($,~)
endfunction

function a($)
    dim $b, $c, $e
    $e = $^0
    while $
        $b = ~$b
        $a = (2+$b) * right($,1)
        $c = $c + $a-9*($a>9)
        $a = ($c mod 10 =) * ($>2&$<7&$e+($<4)=16) * $
        $ = left($,~)
endfunction