Function carry_through_edit 

pub fn carry_through_edit(
    spans: &mut Vec<MarkSpan>,
    at: usize,
    inserted_len: usize,
    removed_len: usize,
)

shift_after_edit the way the document carries marks over typing: a span ending where text is inserted grows over it, as a mark the document stores with ExpandMark::After does. Typing right after bold text goes on in bold, and an editor shows what its document will hold.