Function insert_at 

pub fn insert_at(
    src: &str,
    line: usize,
    col: usize,
    text: &str,
) -> Result<String, EditError>

Inserts text at line (1-based) and col (0-based characters).

Errors

When the line or column is outside the source. The line just after the last one, column 0, means the end of the source.