Function A($)
	dim $s,$f
	$a = 0
	if $>3 & $<7 & len($)+($<4)=16
		while $
			$a = right($,1)
			$s = $s + $f*(-9*(4<$a)+$a) + $a
			$ = left($,-1)
			$f=1-$f
			if $s mod 10 $a =0
Endfunction