diff -r 5d7feca9a1c9 -r d2f2bcf116e8 doc/testing.txt
--- a/doc/testing.txt	Fri Feb 06 14:09:12 2015 -0800
+++ b/doc/testing.txt	Fri Feb 06 15:43:34 2015 -0800
@@ -198,5 +198,13 @@
         '-e "s|$(PERL)|\\$$(PERL)|g" ' \
         '-e "s|$(SOURCE_DIR)|\\$$(SOURCE_DIR)|g" '
 
+Note that in some cases, gsed is not powerful enough.  E.g., if you need to
+do more than massage or remove certain lines, but only compare certain sections
+of the file, then you may need to set COMPONENT_TEST_TRANSFORMER to something
+other than $(GSED) such as $(NAWK).  In this case, you will want to set
+COMPONENT_TEST_TRANSFORMS using "=" instead of "+=", as the default sed
+transforms will likely need to be tossed aside in favor of whatever arguments
+your favorite transformer might need.
+
 When your master test file(s) are in good shape, then you should "hg add"
 them to your workspace.