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    pub trait _StrExt {
        fn _is_char_boundary(&self, index: usize) -> bool;
    }

    impl _StrExt for str {
        #[inline]
        fn _is_char_boundary(&self, index: usize) -> bool {
            if index == self.len() { return true; }
            match self.as_bytes().get(index) {
                None => false,
                Some(&b) => b < 128 || b >= 192,
            }
        }
    }

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