Polish NIP validation

	public static bool ValidateNip(string nip)
	{
	if (nip.Length != 10) return false;
	try { Int64.Parse(nip); }
	catch { return false; }
	int[] weights = new int[] { 6, 5, 7, 2, 3, 4, 5, 6, 7 };
	int sum = 0;
	for (int i = 0; i < weights.Length; i++)
	sum += int.Parse(nip.Substring(i, 1)) * weights[i];
	return (sum % 11) == int.Parse(nip.Substring(9, 1));
	}

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