message: the index public derivation of the change public key cannot fail
  relates to
diff: @@ -250,7 +250,7 @@ impl scheme::Account for Account<XPub> {
  for addressing in addresses {
  let key = self.cached_root_key
  .change(self.derivation_scheme, addressing.0).expect("cannot fail")
  - .index(self.derivation_scheme, addressing.1).unwrap();
  + .index(self.derivation_scheme, addressing.1).expect("cannot fail");
  let addr = ExtendedAddr::new_simple(key.0);
  vec.push(addr);
  }
  