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Predicate http_open_websocket/3
 http_open_websocket(+URL, -WebSocket, +Options) is det
Establish a client websocket connection. This predicate calls http_open/3 with additional headers to negotiate a websocket connection. In addition to the options processed by http_open, the following options are recognised:
List of subprotocols that are acceptable. The selected protocol is available as ws_property(WebSocket, subprotocol(Protocol).

The following example exchanges a message with the html5rocks.websocket.org echo service:

?- URL = 'ws://html5rocks.websocket.org/echo',
   http_open_websocket(URL, WS, []),
   ws_send(WS, text('Hello World!')),
   ws_receive(WS, Reply),
   ws_close(WS, 1000, "Goodbye").
URL = 'ws://html5rocks.websocket.org/echo',
WS = <stream>(0xe4a440,0xe4a610),
Reply = websocket{data:"Hello World!", opcode:text}.
WebSocket- is a stream pair (see stream_pair/3)