Add expand-barrier and skip-child-expansion

It's going to be like TeX before you know it... ._.

* src/current/include/preproc/package.xsl (preproc:tpl-check)
  [lv:template|lv:const|lv:typedef|lv:param-copy]: Add lv:param-copy.
* src/current/include/preproc/template.xsl (preproc:apply-template)
 [lv:expand-barrier, lv:skip-child-expansion]: New expansion control
   structures.

src/current/include/preproc/package.xsl

@@ -259,7 +259,7 @@
 
 
 <!-- skip things that cannot contain template applications -->
-<xsl:template match="lv:template|lv:const|lv:typedef"
+<xsl:template match="lv:template|lv:const|lv:typedef|lv:param-copy"
               mode="preproc:tpl-check" priority="9">
 </xsl:template>
 

src/current/include/preproc/template.xsl

@@ -584,6 +584,27 @@
 </xsl:template>
 
 
+<!--
+  Halt expansion at this point and remove the barrier for future expansions
+-->
+<xsl:template match="lv:expand-barrier" mode="preproc:apply-template" priority="5">
+  <xsl:sequence select="node()" />
+</xsl:template>
+
+
+<!--
+  Leave immediate children alone but continue expanding grandchildren
+-->
+<xsl:template match="lv:skip-child-expansion" mode="preproc:apply-template" priority="5">
+  <xsl:for-each select="element()">
+    <xsl:copy>
+      <xsl:copy-of select="@*" />
+      <xsl:apply-templates mode="preproc:apply-template" />
+    </xsl:copy>
+  </xsl:for-each>
+</xsl:template>
+
+
 <!--
   Preprocesses template nodes