+/**
+ * The `Filter` trait defines behavior for indicating chain activity of interest pertaining to
+ * channels.
+ *
+ * This is useful in order to have a [`Watch`] implementation convey to a chain source which
+ * transactions to be notified of. Notification may take the form of pre-filtering blocks or, in
+ * the case of [BIP 157]/[BIP 158], only fetching a block if the compact filter matches. If
+ * receiving full blocks from a chain source, any further filtering is unnecessary.
+ *
+ * After an output has been registered, subsequent block retrievals from the chain source must not
+ * exclude any transactions matching the new criteria nor any in-block descendants of such
+ * transactions.
+ *
+ * Note that use as part of a [`Watch`] implementation involves reentrancy. Therefore, the `Filter`
+ * should not block on I/O. Implementations should instead queue the newly monitored data to be
+ * processed later. Then, in order to block until the data has been processed, any [`Watch`]
+ * invocation that has called the `Filter` must return [`TemporaryFailure`].
+ *
+ * [`TemporaryFailure`]: ChannelMonitorUpdateErr::TemporaryFailure
+ * [BIP 157]: https://github.com/bitcoin/bips/blob/master/bip-0157.mediawiki
+ * [BIP 158]: https://github.com/bitcoin/bips/blob/master/bip-0158.mediawiki
+ */