--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/mercurial/destutil.py	Mon Oct 05 01:46:47 2015 -0700
@@ -0,0 +1,56 @@
+# destutil.py - Mercurial utility function for command destination
+#
+#  Copyright Matt Mackall <mpm@selenic.com> and other
+#
+# This software may be used and distributed according to the terms of the
+# GNU General Public License version 2 or any later version.
+
+from .i18n import _
+from . import (
+    error,
+    util,
+    obsolete,
+)
+
+def destupdate(repo):
+    """destination for bare update operation
+    """
+    # Here is where we should consider bookmarks, divergent bookmarks, and tip
+    # of current branch; but currently we are only checking the branch tips.
+    node = None
+    wc = repo[None]
+    p1 = wc.p1()
+    try:
+        node = repo.branchtip(wc.branch())
+    except error.RepoLookupError:
+        if wc.branch() == 'default': # no default branch!
+            node = repo.lookup('tip') # update to tip
+        else:
+            raise util.Abort(_("branch %s not found") % wc.branch())
+
+    if p1.obsolete() and not p1.children():
+        # allow updating to successors
+        successors = obsolete.successorssets(repo, p1.node())
+
+        # behavior of certain cases is as follows,
+        #
+        # divergent changesets: update to highest rev, similar to what
+        #     is currently done when there are more than one head
+        #     (i.e. 'tip')
+        #
+        # replaced changesets: same as divergent except we know there
+        # is no conflict
+        #
+        # pruned changeset: no update is done; though, we could
+        #     consider updating to the first non-obsolete parent,
+        #     similar to what is current done for 'hg prune'
+
+        if successors:
+            # flatten the list here handles both divergent (len > 1)
+            # and the usual case (len = 1)
+            successors = [n for sub in successors for n in sub]
+
+            # get the max revision for the given successors set,
+            # i.e. the 'tip' of a set
+            node = repo.revs('max(%ln)', successors).first()
+    return repo[node].rev()