Select a version on a document to compare with current one

As an user with read permissions I want to choose a version to compare

Functional description

When viewing a document, on the version widget, the user can list versions. The user should be able to compare the viewing version to any of the versions on the widget (minus the current one).
If the user is currently viewing a specific version, that version should be the on to compare against. Also, the "after/right" version will be the selected one, but all versions should be available on the comparison interface (drop downs).

If the user is not viewing a specific version, the comparison should be with the Live version (Live should also be part of the version set).