Function set_heading 

pub fn set_heading(
    content: &mut Element,
    path: &[usize],
    level: Option<u8>,
    styles: &mut Styles,
) -> Result<(), Refused>

Make a paragraph a heading of a level, or body text again.

A paragraph that is already what is asked for is left alone. The paragraph’s own style is replaced, which is what applying a paragraph style is; its text, its spans and everything in it stay.

A document with no office:automatic-styles is given one before its body when a heading style has to be written, which moves the body one place along: a path held from before the edit is to be taken again.

Errors

The path does not lead to a paragraph, or the content root does not declare a namespace the edit writes in. Each is refused before anything changes.