Clarification on :::= Variable Assignment Operator?

Several places in variable.c and variable.h mention the
':::=' variable assignment operator, and describe it as
a Posix addition. However, I can't find any mention of it
anywhere on the Internet or in the Gmake manual.

What I did find were lots of mentions of the '::=' operator.
Paul S. mentioned it in the Gmake 4.0 announcement.

"New feature: "::=" simple assignment operator as defined by POSIX in 2012."

And the Gmake manual says:

"Variable definitions are parsed as follows:
	immediate = deferred
	immediate ?= deferred
	immediate := immediate
	immediate ::= immediate
	immediate += deferred or immediate
	immediate != immediate"

Is ':::=' a typo?

Thanks,
Jon