ist). This is useful if you need
    to access the version objects multiple times, because they do not have to
    be recreated this way.

    Examples ('package.versions' being a version list):
        '0.7.92' in package.versions # Check whether 0.7.92 is a valid version.
        package.versions[0] # Return first version or raise IndexError
        package.versions[0:2] # Return a new VersionList for objects 0-2
        package.versions['0.7.92'] # Return version 0.7.92 or raise KeyError
        package.versions.keys() # All keys, as strings.
        max(package.versions)
    Nc