Filters: negated label expressions don't appear to be supported

The engine docs specify that you can negate label expressions:

• label (label=<key>, label=<key>=<value>, label!=<key>, or label!=<key>=<value>) Prune networks with (or without, in case label!=... is used) the specified labels.

Frankly, the engine docs leave a lot to be desired, there's no example of a negated mapping, so I don't know what format the engine expects. docker.utils.utils.convert_filters() just formats the filters as a JSON map of filter name (e.g. label) to a list of label expressions. I tried running client.prune_networks(filters={'label': '!=foo=bar'}) and no networks were deleted, despite me having created two networks, one with foo=bar and the other with foo=baz.

Am I just going about this wrong, or are negative expressions not supported?

Start with docker.utils.utils.convert_filters() and client.prune_networks(), then compare their filter format with the linked Docker Engine network-prune documentation. Reproduce the case using networks labeled foo=bar and foo=baz. Done means determining the expected negated-label format and whether docker-py supports it, with the behavior or documentation clarified accordingly.