How much testing is enough?

Well since the stylesheet is more generic I guess if it makes sure
that h2 h4 returns an error then the error checking works.

The David code needs to be improved with more exact error messages,

An <xsl:value-of select="concat('h',$h[current()])"/> cannot be
followed directly by a <xsl:value-of select="concat('h',$h[current() +
1])"/> the rule is -- state the rule here

and we also have the problem of transferring a for-each to Schematron.
I guess we need to make an assertion for each heading level.

Cheers,
Bryan Rasmussen