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