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I had to use `impl<'a, V: Copy> Extend<(usize, &'a V)> for VecMap<V>` instead of `impl<'a, V: Copy> Extend<(&'a usize, &'a V)> for VecMap<V>` as that's what is needed for doing ```rust let mut a = VecMap::new(); let b = VecMap::new(); b.insert(1, "foo"); a.extend(&b) ``` I can squash the commits after review. r? @Gankro