Is ::= a typo or actually an operator?

[Regulus]

I am running python 3.4, and trying to learn how to use the keyword assert. It's featured on the Pythong docs page https://docs.python.org/3/reference/simp...-statement, section 7.3.

This line is giving me a syntax error:
assert_stmt ::= "assert" expression1 ["," expression2]

Is ::= an operator or a typo? The compiler highlights the first colon when I run it.