Function A($)

	
	dim $i, $x, $b, $l
	$l = len($)

	if $l = 15 or $l = 16
		redim $b[$l-1]
		for $i = 0 to $l-1
			$b[$i] = substr($,$i+1,1)
		next

		for $i = $l-2 to 0 step -2
			$x = 2*$b[$i]
			$b[$i] = iif($x>9,0+left($x,1) + right($x,1),$x)
		next

		$i = execute("$X="+join($b,'+'))
			
		if right($x,1) = 0
			$B = left($,1)
			if $b = 3
				if $l <> 15
					$b = 0
				endif
			else
				if instr('456',$b)
					if $l <> 16
						$b = 0
					endif	
				else
					$b = 0
				endif
			endif
		else
			$b = 0
		endif
	else
		$b = 0
	endif
	$a = $b


Endfunction