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    /// An enum signifying that some of type `T` is allowed, or `All` (anything is allowed).
    #[derive(Debug, Clone, PartialEq, Eq)]
    pub enum AllOrSome<T> {
        /// Everything is allowed. Usually equivalent to the `*` value.
        All,

        /// Only some of `T` is allowed
        Some(T),
    }

    /// Default as `AllOrSome::All`.
    impl<T> Default for AllOrSome<T> {
        fn default() -> Self {
            AllOrSome::All
        }
    }

    impl<T> AllOrSome<T> {
        /// Returns whether this is an `All` variant.
        pub fn is_all(&self) -> bool {
            matches!(self, AllOrSome::All)
        }

        /// Returns whether this is a `Some` variant.
        #[allow(dead_code)]
        pub fn is_some(&self) -> bool {
            !self.is_all()
        }

        /// Provides a shared reference to `T` if variant is `Some`.
        pub fn as_ref(&self) -> Option<&T> {
            match *self {
                AllOrSome::All => None,
                AllOrSome::Some(ref t) => Some(t),
            }
        }

        /// Provides a mutable reference to `T` if variant is `Some`.
        pub fn as_mut(&mut self) -> Option<&mut T> {
            match *self {
                AllOrSome::All => None,
                AllOrSome::Some(ref mut t) => Some(t),
            }
        }
    }

    #[cfg(test)]
    #[test]
    fn tests() {
        assert!(AllOrSome::<()>::All.is_all());
        assert!(!AllOrSome::<()>::All.is_some());

        assert!(!AllOrSome::Some(()).is_all());
        assert!(AllOrSome::Some(()).is_some());
    }