code: def psubscribe(pattern, proc = nil, &block)
        if cb = proc || block
          @psub_callbacks[pattern] << cb
        end
        @psubs << pattern
        raw_send_command(:psubscribe, [pattern])
        return pubsub_deferrable(pattern)
      end
docs: Pattern subscribe to a pubsub channel
  
   If an optional proc / block is provided then it will be called (with the
   channel name and message) when a message is received on a matching
   channel
  
   @return [Deferrable] Redis psubscribe call