From PR 232042
188.8.131.52. A Simple Gateway with NAT, in the first paragraph:
xl1 <----> xl0
184.108.40.206. A Simple Gateway with NAT, between the variables of /etc/rc.conf and the first filtering rules:
allows stateful traffic to pass from the Internet to hosts on the network ----> allows stateful traffic from hosts of the internal network to pass to the gateway
N.B. These errors are confirmed by the translation rule found further below and the subsequent explanation.