Appendix B. Syntax diagrams.

The rules that apply to the railroad track syntax diagrams are as follows:

• Read the syntax diagrams from left to right, from top to bottom, following the path of the line.

  The >>-- symbol indicates the beginning of a statement.
  The --> symbol indicates that the statement syntax is continued on the next line.
  The >-- symbol indicates that a statement is continued from the previous line.
  The -->< symbol indicates the end of a statement.
  Diagrams of syntactical units other that complete statements start with the >-- symbol and end with the --> symbol.

• Required items appear on the horizontal line (the main path).

     >>---STATEMENT---required_item------------------------------><
  

• Optional items appear below the main path.

     >>---STATEMENT--+-----------------+-------------------------><
                     +--optional_item--+
  

• If you can choose from two or more items, they appear vertically, in a stack.  If you must choose one of the items, one item of the stack appears on the main path.

     >>---STATEMENT--+-required-choice1-+------------------------><
                     +-required-choice2-+
  

  If choosing one of the items is optional, the entire stack appears below the main path.

     >>---STATEMENT--+------------------+------------------------><
                     !-optional-choice1-!
                     +-optional_choice2-+
  

• If one of the items is the default, it will appear above the main path and the remaining choices will be shown below.

                     +--default_choice--+
     >>---STATEMENT--!------------------!------------------------><
                     !-optional-choice1-!
                     +-optional_choice2-+
  

• An arrow returning to the left above the main line indicates an item that can be repeated.

                     +-----------------+
                     v                 !
     >>---STATEMENT----repeatable-item-+-------------------------><
  

  A repeat arrow above a stack indicates that you can repeat the items in the stack.

• Keywords appear in uppercase, for example, PARM1, must be spelled exactly as shown.  Variables appear in all lowercase letters (for example, parmx).  They represent user-supplied names or values.

• If punctuation marks, parenthesis, arithmetic operators, or such symbols are shown, you must enter them as part of the syntax.