Currently crypto::mem::Encrypted's map function signature looks like this: ```rust pub fn map<F, T>(&self, mut fun: F) -> T where F: FnMut(&Protected) -> T { ... } ``` It only calls F once, it would be better to replace FnMut with FnOnce to make it work with more closures. Since FnOnce is a supertrait of FnMut, it would not even be a breaking change.