Perform a bit of html validation for PGML tag blocks that are spans.

A span tag block is not allowed to contain any PGML blocks that have the
type indent, align, par, list, bullet, answer, heading, rule, code, pre,
verbatim, table, or tag.  If a span tag block is detected with any of
those things in it, a warning is issued and the contents rendered
directly without the span tag.  This helps to prevent problem authors
from doing the wrong thing.

It is valid for a span to contain a span, but if a span tag block
contains another tag bloc, the outer span tag block does not have the
information to determine what the inner tag block is (span or div).
Since I don't think it is particularly useful to have a span within a
span, I just blocked any tag block in a span.

macros/core/PGML.pl

@@ -1658,6 +1658,15 @@ sub Tag {
 		PGML::Warning qq{The tag "$tag" is not allowed};
 		return $self->string($item);
 	}
+	if ($tag eq 'span') {
+		for my $subblock (@{ $item->{stack} }) {
+			if ($subblock->{type} =~ /^(indent|align|par|list|bullet|answer|heading|rule|code|pre|verbatim|table|tag)$/)
+			{
+				PGML::Warning qq{A "span" tag may not contain a $subblock->{type}};
+				return $self->string($item);
+			}
+		}
+	}
 	return main::tag($tag, @attributes, $self->string($item));
 }