A naive example

Consider a well-formatted document containing no mathematical formulae. Here, the layout structure consists of a root node, which is the page, and the paragraphs which are the various children. At the next level on this tree, we have the lines, and each line is further broken up into words and words themselves are broken up into characters. Given this structure, we can rephrase all of the browsing actions as a combination of simple tree traversal movements. Thus, we can identify the following atomic actions:

1. Go to next sibling.
2. Go to previous sibling.
3. Go to parent.
4. Go to left most child.
5. Go to rightmost child.
6. Mark current node.
7. Return to marked node.

Using the above atomic actions and their various combinations, we can define all the browsing actions that the eye is capable of performing.

Thus, on encountering a reference to a footnote while reading we:

1. Mark current node.
2. Go to parent (this gets us out of the current paragraph).
3. Move across siblings until footnote is located.
4. Read footnote.
5. Return to marked node.