I got a sample code:

let s ="abcd";letmut cursor =GraphemeCursor::new(3, s.len(),false);assert_eq!(cursor.prev_boundary(&s[2..],2),Err(GraphemeIncomplete::PrevChunk));assert_eq!(cursor.prev_boundary(&s[..2],0),Ok(Some(2)));

On the second assertion, it instead returnsErr(GraphemeIncomplete::InvalidOffset). If it starts from position 4, as is written in the doc comment, it works. However it does not work when the cursor position starts from 3.