diff --git a/xml/upgrade-3.10-common.xsl b/xml/upgrade-3.10-common.xsl
index 63ae2a7120..0821ff3673 100644
--- a/xml/upgrade-3.10-common.xsl
+++ b/xml/upgrade-3.10-common.xsl
@@ -1,31 +1,39 @@
 <?xml version="1.0" encoding="UTF-8"?>
 
 <!--
  Use comments liberally as future maintainers may be unfamiliar with XSLT.
  -->
 
 <!--
  upgrade-3.10-common.xsl
 
  This stylesheet is intended to be imported by all other stylesheets in the
  upgrade-3.10-* pipeline. It provides variables and templates that are used by
  multiple stylesheets.
 
  This file should not contain any templates with a match attribute.
+
+ Assumptions:
+ * No element of the input XML contains an id attribute whose value begins with
+   "pcmk__3_10_upgrade-". This allows us to generate new IDs without fear of
+   conflict. However, the schema does not enforce this assumption.
  -->
 
 <xsl:stylesheet version="1.0"
                 xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
 
 <!-- Strip whitespace-only text nodes but indent output -->
 <xsl:strip-space elements="*"/>
 <xsl:output encoding="UTF-8" indent="yes" omit-xml-declaration="yes"/>
 
+<!-- Prefix for auto-generated IDs -->
+<xsl:variable name="upgrade_prefix" select="'pcmk__3_10_upgrade-'"/>
+
 <!-- Identity transformation: copy everything unaltered by default -->
 <xsl:template name="identity">
     <xsl:copy>
         <xsl:apply-templates select="@*|node()"/>
     </xsl:copy>
 </xsl:template>
 
 </xsl:stylesheet>