Not a typo.

http://www.tom-yam.or.jp/2238/src/slp.c.html#line2238

It's what we'd use `&=` for nowadays. Note that the "B" language had these sorts of operators around the opposite way for how they appeared in "C" (e.g. read https://www.bell-labs.com/usr/dmr/www/kbman.html).