The form that is used for indexing. This could be '0002936X' or '0002936' because hyphen and check digit do not contain information. You could also store the ISSN in the 32 bit integer number '2996' instead of a string.

### Example: VIAF-ID package SeeAlso::Identifier::VIAF;

use base qw(SeeAlso::Identifier);

sub parse { my $value = shift; return '' unless $value =~ /^\s*(http:\/\/viaf.org\/)?([0-9]+)\s*$/; return "http://viaf.org/$2"; }

# return local id only sub hash { return $_[0] eq '' ? substr( $_[0], 16 ) : ''; }

prefix => 'http://viaf.org/'