rust iter3

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struct CountdownIterator(i32);   impl Iterator for CountdownIterator {     type Item = i32;     fn next(&mut self) -> Option<Self::Item> {         self.0 -= 1;         if self.0 < 0 {             None         } else {             Some(self.0)         }     } }   struct Countdown(i32);   impl IntoIterator for Countdown {     type Item = i32;     type IntoIter = CountdownIterator;     fn into_iter(self) -> Self::IntoIter {         CountdownIterator(self.0)     } }   impl<'a> IntoIterator for &'a Countdown {     type Item = i32;     type IntoIter = CountdownIterator;     fn into_iter(self) -> Self::IntoIter {         CountdownIterator(self.0)     } }   impl<'a> IntoIterator for &'a mut Countdown {     type Item = i32;     type IntoIter = CountdownIterator;     fn into_iter(self) -> Self::IntoIter {         CountdownIterator(self.0)     } }   fn main() {     let c = Countdown(10);     for i in &c {         for j in &c {             println!("({0}, {1})", i, j)         }     } }