Perl extension for International Standard Serial Numbers
The constructor accepts a scalar representing the ISSN.
The string representing the ISSN may contain characters other than
[0-9xX], although these will be removed in the internal representation.
The resulting string must look like an ISSN - the first seven characters
must be digits and the eighth character must be a digit, 'x', or 'X'.
The string passed as the ISSN need not be a valid ISSN as long as it
superficially looks like one. This allows one to use the 'fix_checksum'
One should check the validity of the ISSN with 'is_valid()' rather than
relying on the return value of the constructor.
If all one wants to do is check the validity of an ISSN, one can skip the
object-oriented interface and use the c<is_valid_checksum()> function
which is exportable on demand.