Watches for a particular stream of actions in the specified order and emits a list of those actions once all sequence steps have been matched. This will find the first occurence of the substream and not necessarily the most concise or recent substream. For example, substream([A, B]) encountering an action stream of [A1, A2, B1, B2] will emit [A1, B1]. When a substream is found and emitted, the list is reset and it will continue to listen to the action stream. You can indicate that a particular step in the sequence can be matched by mutiple actions by passing an array for that step. For example, if the first step of the sequence could be action A or action B: substream([[A, B], C]). An action stream of [A1, C1] would cause substream to emit [A1, C1]. An action stream of [B1, C1] would cause substream to emit [B1, C1]. An action stream of [A1, B1, C1] would cause substream to emit [A1, C1]. You can optionally pass a list of terminators. If a terminator is encountered in the action stream, any partially matched substream is reset. For example, substream([A, B], C) would not emit anything for an action stream of [A1, C1, B1].