Function group 

pub fn group<'a, Id, Message, Theme, Renderer>(
    id: Id,
    header: impl IntoFragment<'a>,
    content: impl Fn(Size) -> Option<Element<'a, Message, Theme, Renderer>> + 'a,
) -> Group<'a, Id, Message, Theme, Renderer>
where
    Id: Clone + Eq,
    Message: 'a + Clone,
    Theme: 'a + Catalog + Catalog + Catalog,
    <Theme as Catalog>::Class<'a>: From<StyleFn<'a, Theme>>,
    <Theme as Catalog>::Class<'a>: From<StyleFn<'a, Theme>>,
    Renderer: 'a + Renderer + Renderer + Renderer,
    <Renderer as Renderer>::Font: From<Font>,
    <Renderer as Renderer>::Paragraph: Clone,

Available on crate feature ribbon only.