rust/protover: delete ProtoSet::retain

        false

    }

    /// Retain only the `Version`s in this `ProtoSet` for which the predicate

    /// `F` returns `true`.

    ///

    /// # Examples

    ///

    /// ```

    /// # use protover::errors::ProtoverError;

    /// use protover::protoset::ProtoSet;

    ///

    /// # fn do_test() -> Result<bool, ProtoverError> {

    /// let mut protoset: ProtoSet = "1,3-5,9".parse()?;

    ///

    /// // Keep only versions less than or equal to 8:

    /// protoset.retain(|x| x <= &8);

    ///

    /// assert_eq!(protoset.expand(), vec![1, 3, 4, 5]);

    /// #

    /// # Ok(true)

    /// # }

    /// # fn main() { do_test(); }  // wrap the test so we can use the ? operator

    /// ```

    // XXX we could probably do something more efficient here. —isis

    pub fn retain<F>(&mut self, f: F)

        where F: FnMut(&Version) -> bool

    {

        let mut expanded: Vec<Version> = self.clone().expand();

        expanded.retain(f);

        *self = expanded.into();

    }

    /// Returns all the `Version`s in `self` which are not also in the `other`

    /// `ProtoSet`.

    ///