$ hg init repo

  $ cd repo

  $ echo 0 > a

  $ hg ci -qAm 0

  >   hg up -q 0

  >   echo $i > a

  >   hg ci -qm $i

  > done

  $ cat <<EOF >> .hg/hgrc

  > [alias]

  > l = log -T '{rev}:{shortest(node,1)}\n'

  > EOF

  $ hg l

  4:7ba5d

  3:7ba57

  2:72

  1:9

  0:b

  $ cat <<EOF >> .hg/hgrc

  > [experimental]

  > EOF

  $ hg l

  4:7ba5d

  3:7b

  2:72

  1:9

  0:b

9 was unambiguous and still is

  $ hg l -r 9

  1:9

7 was ambiguous and still is

  $ hg l -r 7

  abort: 00changelog.i@7: ambiguous identifier!

  [255]

7b is no longer ambiguous

  $ hg l -r 7b

  3:7b

  $ cd ..