--- a/mercurial/commands.py	Fri Jul 14 14:51:36 2006 -0700
+++ b/mercurial/commands.py	Fri Jul 14 23:19:15 2006 -0700
@@ -865,11 +865,22 @@
     if op2 != nullid:
         raise util.Abort(_('outstanding uncommitted merge'))
     node = repo.lookup(rev)
-    parent, p2 = repo.changelog.parents(node)
-    if parent == nullid:
+    p1, p2 = repo.changelog.parents(node)
+    if p1 == nullid:
         raise util.Abort(_('cannot back out a change with no parents'))
     if p2 != nullid:
-        raise util.Abort(_('cannot back out a merge'))
+        if not opts['parent']:
+            raise util.Abort(_('cannot back out a merge changeset without '
+                               '--parent'))
+        p = repo.lookup(opts['parent'])
+        if p not in (p1, p2):
+            raise util.Abort(_('%s is not a parent of %s' %
+                               (short(p), short(node))))
+        parent = p
+    else:
+        if opts['parent']:
+            raise util.Abort(_('cannot use --parent on non-merge changeset'))
+        parent = p1
     repo.update(node, force=True, show_stats=False)
     revert_opts = opts.copy()
     revert_opts['rev'] = hex(parent)
@@ -2829,6 +2840,7 @@
           ('m', 'message', '', _('use <text> as commit message')),
           ('l', 'logfile', '', _('read commit message from <file>')),
           ('d', 'date', '', _('record datecode as commit date')),
+          ('', 'parent', '', _('parent to choose when backing out merge')),
           ('u', 'user', '', _('record user as committer')),
           ('I', 'include', [], _('include names matching the given patterns')),
           ('X', 'exclude', [], _('exclude names matching the given patterns'))],

--- a/tests/test-backout	Fri Jul 14 14:51:36 2006 -0700
+++ b/tests/test-backout	Fri Jul 14 23:19:15 2006 -0700
@@ -60,4 +60,40 @@
 hg backout -d '3 0' 1
 hg locate b
 
+cd ..
+hg init m
+cd m
+echo a > a
+hg commit -d '0 0' -A -m a
+echo b > b
+hg commit -d '1 0' -A -m b
+echo c > c
+hg commit -d '2 0' -A -m b
+hg update 1
+echo d > d
+hg commit -d '3 0' -A -m c
+hg merge 2
+hg commit -d '4 0' -A -m d
+
+echo '# backout of merge should fail'
+
+hg backout 4
+
+echo '# backout of merge with bad parent should fail'
+
+hg backout --parent 0 4
+
+echo '# backout of non-merge with parent should fail'
+
+hg backout --parent 0 3
+
+echo '# backout with valid parent should be ok'
+
+hg backout -d '5 0' --parent 2 4
+
+hg rollback
+hg update -C
+
+hg backout -d '6 0' --parent 3 4
+
 exit 0