Bug in remove_prefix?#22

The following test seems to produce wrong result: ``` rust #[test] fn remove_prefix() { let mut trie = Trie::new(); for i in 0..10 { let mut bytes = [0; 16]; let (left, right) = bytes.split_at_mut(8); left.copy_from_slice(&u64::to_be_bytes(i)); right.copy_from_slice(&u64::to_be_bytes(i)); println!("insert {:?}", bytes); trie.insert(bytes, ()); } println!("trie = {:?}", trie); assert_eq!(trie.count(), 10); for i in 0..5 { let subtrie = trie.remove_prefix(&u64::to_be_bytes(i)[..]); println!("remove_prefix {} removed {}", i, subtrie.count()); } println!("trie = {:?}", trie); assert_eq!(trie.count(), 5); } ``` The output is: ``` running 1 test test remove_prefix ... FAILED failures: ---- remove_prefix stdout ---- test remove_prefix ... FAILED failures: ---- remove_prefix stdout ---- insert [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0] insert [0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 1] insert [0, 0, 0, 0, 0, 0, 0, 2, 0, 0, 0, 0, 0, 0, 0, 2] insert [0, 0, 0, 0, 0, 0, 0, 3, 0, 0, 0, 0, 0, 0, 0, 3] insert [0, 0, 0, 0, 0, 0, 0, 4, 0, 0, 0, 0, 0, 0, 0, 4] insert [0, 0, 0, 0, 0, 0, 0, 5, 0, 0, 0, 0, 0, 0, 0, 5] insert [0, 0, 0, 0, 0, 0, 0, 6, 0, 0, 0, 0, 0, 0, 0, 6] insert [0, 0, 0, 0, 0, 0, 0, 7, 0, 0, 0, 0, 0, 0, 0, 7] insert [0, 0, 0, 0, 0, 0, 0, 8, 0, 0, 0, 0, 0, 0, 0, 8] insert [0, 0, 0, 0, 0, 0, 0, 9, 0, 0, 0, 0, 0, 0, 0, 9] trie = {[0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]: (), [0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 1]: (), [0, 0, 0, 0, 0, 0, 0, 2, 0, 0, 0, 0, 0, 0, 0, 2]: (), [0, 0, 0, 0, 0, 0, 0, 3, 0, 0, 0 , 0, 0, 0, 0, 3]: (), [0, 0, 0, 0, 0, 0, 0, 4, 0, 0, 0, 0, 0, 0, 0, 4]: (), [0, 0, 0, 0, 0, 0, 0, 5, 0, 0, 0, 0, 0, 0, 0, 5]: (), [0, 0, 0, 0, 0, 0, 0, 6, 0, 0, 0, 0, 0, 0, 0, 6]: (), [0, 0, 0, 0, 0, 0, 0, 7, 0, 0, 0, 0, 0, 0, 0, 7]: (), [0, 0, 0, 0, 0, 0, 0, 8, 0, 0, 0, 0, 0, 0, 0, 8]: (), [0, 0, 0, 0, 0, 0, 0, 9, 0, 0, 0, 0, 0, 0, 0, 9]: ()} remove_prefix 0 removed 10 remove_prefix 1 removed 0 remove_prefix 2 removed 0 remove_prefix 3 removed 0 remove_prefix 4 removed 0 trie = {} thread 'remove_prefix' panicked at 'assertion failed: `(left == right)` left: `10`, right: `5`', tests/lib.rs:317:5 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace ``` There are at least two problems: - Each remove_prefix should return a subtree that has only 1 element. But the first one says "removed 10", the rest says "0". - After remove_prefix, the count is still 10 despite that all elements are removed.