diff --git a/make-includes/standard_rules.make b/make-includes/standard_rules.make
index 16e331c..f4be5f0 100755
--- a/make-includes/standard_rules.make
+++ b/make-includes/standard_rules.make
@@ -48,8 +48,22 @@
 #
 # Derived XML with inclusions from source XML without inclusions.
 #
+# This is done by simply running the original source through xmllint
+# with the --xinclude option to process any xi:include elements.
+# The result of this could just be piped into the XSLT processor,
+# except that (a) not all of the processors support input from stdin,
+# and (b) combining both xmllint and XSLT processing into one command
+# means that make won't stop if there's any errors from xmllint.
+#
+# Sed is used to add a comment to the derived XML file, warning that this
+# is generated and shouldn't be edited. Sed was used because the <?xml?>
+# processing instruction MUST be on the first line. The implication here
+# is that all input source files have an <?xml?> processing instruction
+# (which they should anyway).
+#
 %-derived.xml: %.xml
 	xmllint --xinclude $< > $@
+	@sed -i -e '1a <!-- THIS FILE IS AUTOMATICALLY GENERATED. DO NOT EDIT! -->' $@
 #
 # LaTeX from derived XML.
 #