Why is ::= used as assignment operator in doc statements?

[Colin J. Williams]

Ben Finney wrote:
> schaffe at gmail.com writes:
> 
>> Why is statments expressed with ::= as the assigning operator?
>> like: http://docs.python.org/ref/grammar.txt
> 
> That page is part of the language reference. It shows, not Python
> code, but a description of part of the Python grammar. See §1.2
> <URL:http://www.python.org/doc/ref/notation.html>.
> 
  Of historic interest is Extended 
Bachus-Naur Normal Form (EBNF): 
http://en.wikipedia.org/wiki/Extended_Backus%E2%80%93Naur_form

Colin W.