/// <summary>
/// Converts the ISBN13 to ISBN10.
/// </summary>
/// <param name="isbn">The isbn.</param>
/// <returns></returns>
internal static string ConvertISBN13ToISBN10(string isbn)
{
// Remove the prefix
isbn = isbn.Substring(3, isbn.Length - 3);
// Switch the checksums
var checksum = Spec.CalculateChecksum10(isbn);
isbn = isbn.Substring(0, isbn.Length - 1);
isbn += checksum;
return(isbn);
}