ElementTree addHeading(ElementTree parent, Int headinglevel, String initialtext=createString(0))
parent: The parent element
headinglevel: The level of heading (1-6)
initialtext: Optionally, any initial text the element should contain
Add a heading to the end of the specified element. Heading levels go from 1 to 6, and form a document outline - there should generally be one level 1 heading in the document, with level 2 headings below that, level 3 headings below the level 2 headings, and so on. In practice, headings less important than level 4 are very rarely used.