+/**
+ * The `Watch` trait defines behavior for watching on-chain activity pertaining to channels as
+ * blocks are connected and disconnected.
+ *
+ * Each channel is associated with a [`ChannelMonitor`]. Implementations of this trait are
+ * responsible for maintaining a set of monitors such that they can be updated accordingly as
+ * channel state changes and HTLCs are resolved. See method documentation for specific
+ * requirements.
+ *
+ * Implementations **must** ensure that updates are successfully applied and persisted upon method
+ * completion. If an update fails with a [`PermanentFailure`], then it must immediately shut down
+ * without taking any further action such as persisting the current state.
+ *
+ * If an implementation maintains multiple instances of a channel's monitor (e.g., by storing
+ * backup copies), then it must ensure that updates are applied across all instances. Otherwise, it
+ * could result in a revoked transaction being broadcast, allowing the counterparty to claim all
+ * funds in the channel. See [`ChannelMonitorUpdateErr`] for more details about how to handle
+ * multiple instances.
+ *
+ * [`PermanentFailure`]: ChannelMonitorUpdateErr::PermanentFailure
+ */