-/// let listeners = vec![
-/// (monitor_block_hash, &mut monitor_listener as &mut dyn chain::Listen),
-/// (manager_block_hash, &mut manager as &mut dyn chain::Listen),
-/// ];
+/// let mut listeners = vec![];
+/// if let Some(monitor_block_hash) = monitor_block_hash_option {
+/// listeners.push((monitor_block_hash, &mut monitor_listener as &mut dyn chain::Listen))
+/// }
+/// if let Some(manager_block_hash) = manager_block_hash_option {
+/// listeners.push((manager_block_hash, &mut manager as &mut dyn chain::Listen))
+/// }