Tests for section nesting

H20 -> H19 -> H18 (size nesting)
H20 -> H19 -> H18 -> H19 (size increase in step)
H20 -> H19 -> H19 (sibling alignment)
H20 -> H22 (a situation in which we have to normalise illogically nested tags)
H20 -> H19 -> H18 -> H20 -> H18 (a case that demonstrates the need for the positional stack)