Disallow undeclared NS prefixes?

[dwarring]

Currently permitted, as in:

use LibXML::Element;
my $node = LibXML.load('<Doc/>').root;
my $child = 
$node.setAttribute('xx:y', "42");
my $elem = LibXML::Element.new('yy:Test');
say $elem.Str;
$node.addChild($elem);
say $node.Str; # <Doc xx:y="42"><yy:Test/></Doc>
LibXML.load($node.Str); # illegal xml -dies

This is a carry-over from Perl 5. Not sure if there is a good use case. I'm inclined to disallow it. Can possibly be reenabled as a option, if there really is a need.