ng
    about unpinned packages when he has deeper issues, like VCS
    dependencies, to deal with. Also keeps error reports in a
    deterministic order.
:cvar head: A section heading for display above potentially many
    exceptions of this kind
:ivar req: The InstallRequirement that triggered this error. This is
    pasted on after the exception is instantiated, because it's not
    typically available earlier.

NrN