diff --git a/tests/test-fix.t b/tests/test-fix.t
--- a/tests/test-fix.t
+++ b/tests/test-fix.t
@@ -1713,3 +1713,101 @@
   r0.whole:
   hello
   
+
+We should execute the fixer tools as few times as possible, because they might
+be slow or expensive to execute. The inputs to each execution are effectively
+the file path, file content, and line ranges. So, we should be able to re-use
+results whenever those inputs are repeated. That saves a lot of work when
+fixing chains of commits that all have the same file revision for a path being
+fixed.
+
+  $ hg init numberofinvocations
+  $ cd numberofinvocations
+
+  $ printf "bar1" > bar.log
+  $ printf "baz1" > baz.log
+  $ printf "foo1" > foo.log
+  $ printf "qux1" > qux.log
+  $ hg commit -Aqm "commit1"
+
+  $ printf "bar2" > bar.log
+  $ printf "baz2" > baz.log
+  $ printf "foo2" > foo.log
+  $ hg commit -Aqm "commit2"
+
+  $ printf "bar3" > bar.log
+  $ printf "baz3" > baz.log
+  $ hg commit -Aqm "commit3"
+
+  $ printf "bar4" > bar.log
+
+  $ LOGFILE=$TESTTMP/log
+  $ LOGGER=$TESTTMP/log.py
+  $ cat >> $LOGGER <<EOF
+  > # Appends the input file's name to the log file.
+  > import sys
+  > with open('$LOGFILE', 'a') as f:
+  >     f.write(sys.argv[1] + '\n')
+  > sys.stdout.write(sys.stdin.read())
+  > EOF
+
+  $ hg fix --working-dir -r "all()" \
+  >        --config "fix.log:command=\"$PYTHON\" \"$LOGGER\" {rootpath}" \
+  >        --config "fix.log:pattern=glob:**.log"
+
+  $ cat $LOGFILE | sort | uniq -c
+        4 bar.log
+        4 baz.log
+        4 foo.log
+        4 qux.log
+
+  $ cd ..
+
+For tools that support line ranges, it's wrong to blindly re-use fixed file
+content for the same file revision if it appears twice with different baserevs,
+because the line ranges could be different. Since computing line ranges is
+ambiguous, this isn't a matter of correctness, but it affects the usability of
+this extension. It could maybe be simpler if baserevs were computed on a
+per-file basis to make this situation impossible to construct.
+
+In the following example, we construct two subgraphs with the same file
+revisions, and fix different sub-subgraphs to get different baserevs and
+different changed line ranges. The key precondition is that revisions 1 and 4
+have the same file revision, and the key result is that their successors don't
+have the same file content, because we want to fix different areas of that same
+file revision's content.
+
+  $ hg init differentlineranges
+  $ cd differentlineranges
+
+  $ printf "a\nb\n" > file.changed
+  $ hg commit -Aqm "0 ab"
+  $ printf "a\nx\n" > file.changed
+  $ hg commit -Aqm "1 ax"
+  $ hg remove file.changed
+  $ hg commit -Aqm "2 removed"
+  $ hg revert file.changed -r 0
+  $ hg commit -Aqm "3 ab (reverted)"
+  $ hg revert file.changed -r 1
+  $ hg commit -Aqm "4 ax (reverted)"
+
+  $ hg manifest --debug --template "{hash}\n" -r 0; \
+  > hg manifest --debug --template "{hash}\n" -r 3
+  418f692145676128d2fb518b027ddbac624be76e
+  418f692145676128d2fb518b027ddbac624be76e
+  $ hg manifest --debug --template "{hash}\n" -r 1; \
+  > hg manifest --debug --template "{hash}\n" -r 4
+  09b8b3ce5a507caaa282f7262679e6d04091426c
+  09b8b3ce5a507caaa282f7262679e6d04091426c
+
+  $ hg fix --working-dir -r 1+3+4
+  3 new orphan changesets
+
+  $ hg cat file.changed -r "successors(1)" --hidden
+  a
+  X
+  $ hg cat file.changed -r "successors(4)" --hidden
+  A
+  X
+
+  $ cd ..