$ hg init a

  $ cd a

  $ echo 123 > a

  $ hg add a

  $ hg commit -m "a" -u a

  $ cd ..

  $ hg init b

  $ cd b

  $ echo 321 > b

  $ hg add b

  $ hg commit -m "b" -u b

  $ hg pull ../a

  pulling from ../a

  searching for changes

  abort: repository is unrelated

  [255]

  $ hg pull -f ../a

  pulling from ../a

  searching for changes

  warning: repository is unrelated

  requesting all changes

  adding changesets

  adding manifests

  adding file changes

  added 1 changesets with 1 changes to 1 files (+1 heads)

  (run 'hg heads' to see heads, 'hg merge' to merge)

  $ hg heads

  changeset:   1:9a79c33a9db3

  tag:         tip

  parent:      -1:000000000000

  user:        a

  date:        Thu Jan 01 00:00:00 1970 +0000

  summary:     a

  changeset:   0:01f8062b2de5

  user:        b

  date:        Thu Jan 01 00:00:00 1970 +0000

  summary:     b

  $ cd ..