Function formula_text_edit 

fn formula_text_edit(
    ui: &mut Ui,
    text: &mut String,
    id: Id,
    hint: &str,
    span: Option<Range<usize>>,
    keep_tab: bool,
) -> TextEditOutput

A formula editor: a single-line edit of text with id id, highlighted, the error span underlined, filling the row. keep_tab: Tab stays in the edit rather than moving the focus.