Wondering about circular definitions in Python Language Reference?

If you have been reading the Python Tutorial and did follow the various references to Python Language Reference all the way you likely ended up with something along these lines:

1. enclosure:

   enclosure ::=  parenth_form | list_display | dict_display | set_display
                  | generator_expression | yield_atom
   

2. parenth_form:

   parenth_form ::=  "(" [starred_expression] ")"
   

3. starred_expression:

   starred_expression ::=  expression | ( starred_item "," )* [starred_item]
   

4. starred_item:

   starred_item       ::=  expression | "*" or_expr
   

5. expression:

   expression             ::=  conditional_expression | lambda_expr
   

6. conditional_expression:

   conditional_expression ::=  or_test ["if" or_test "else" expression]
   

   circular definition:

   contitional_expression ::= expression (6)
   expression ::= conditional_expression (5)
   

7. or_test:

   or_test  ::=  and_test | or_test "or" and_test
   

8. and_test:

   and_test ::=  not_test | and_test "and" not_test
   

9. not_test:

   not_test ::=  comparison | "not" not_test
   

10. comparison:

    comparison    ::=  or_expr ( comp_operator or_expr )*
    

11. or_expr:

    or_expr  ::=  xor_expr | or_expr "|" xor_expr
    

12. xor_expr:

    xor_expr ::=  and_expr | xor_expr "^" and_expr
    

13. and_expr:

    and_expr ::=  shift_expr | and_expr "&" shift_expr
    

14. shift_expr:

    shift_expr ::=  a_expr | shift_expr ( "<<" | ">>" ) a_expr
    

15. a_expr:

    a_expr ::=  m_expr | a_expr "+" m_expr | a_expr "-" m_expr
    

16. m_expr:

    m_expr ::=  u_expr | m_expr "*" u_expr | m_expr "@" m_expr |
         m_expr "//" u_expr| m_expr "/" u_expr |
         m_expr "%" u_expr
    

17. u_expr:

    u_expr ::=  power | "-" u_expr | "+" u_expr | "~" u_expr
    

18. power:

    power ::=  ( await_expr | primary ) ["**" u_expr]
    

19. await_expr:

    await_expr ::=  "await" primary
    

20. primary:

    primary ::=  atom | attributeref | subscription | slicing | call
    

21. atom:

    atom      ::=  identifier | literal | enclosure
    

22. enclosure:

    enclosure ::=  parenth_form | list_display | dict_display | set_display
                   | generator_expression | yield_atom
    

And we are back from where we started!

But fear not, there are other similar circular definitions.