The top of a Page Graph.
See Step-By-Step Page Graph for the table of contents.
--- A graph will typically begin with this text:
DOT strict digraph rankdir=LR
The first line indicates the graph uses the DOT 
language. `digraph` means that edges are DIrected (end with arrows). `strict` means that if Page 1 refers twice to Page 2, there's only one arrow between them instead of two.
The second means the graph is laid out left-to-right instead of top-to-bottom.
That graph alone would render as nothing, as shown below:
DOT strict digraph rankdir=LR STATIC strict digraph {rankdir=LR }
Next: Page Graph HERE Node