Function A($)
	dim $i, $x, $b,$s

		for $i = -($^) to -1
			$x = substr($,-$i,1)
			$b = $b+$s*((4<$x)+$x)+$x
			$s=$s^1
			$a = ($>3 & $<7 & ($^)=16-($<4) & $b mod 10=)*left($,1)
Endfunction