+ /**Returns the fee in msats willing to be paid to avoid routing `send_amt_msat` through the
+ * given channel in the direction from `source` to `target`.
+ *
+ * The channel's capacity (less any other MPP parts which are also being considered for use in
+ * the same payment) is given by `channel_capacity_msat`. It may be guessed from various
+ * sources or assumed from no data at all.
+ *
+ * For hints provided in the invoice, we assume the channel has sufficient capacity to accept
+ * the invoice's full amount, and provide a `channel_capacity_msat` of `None`. In all other
+ * cases it is set to `Some`, even if we're guessing at the channel value.
+ *
+ * Your code should be overflow-safe through a `channel_capacity_msat` of 21 million BTC.
+ */