<<.def "Railroad diagrams">>, sometimes called <<.def "syntax diagrams">>, are a visual way of explaining the syntax rules of a computer language. Reading one is like reading a public transport map.
Each diagram starts on the left and ends on the right. Simply follow any line from the startpoint to the endpoint. All the alternative lines are equally valid. A line will sometimes jump over an item that is optional, or loop back to indicate that an item can be repeated.
In the example above, a comma appears between each occurrence of the `repeated` item. The comma path runs from right to left, and can only be reached by first passing through `repeated`.
