problem with anchors in PF

[Mark Nejedlo]

I have a firewall which inserting rules into anchors by calling `echo 
"<rule>" | pfctl -a <anchor> -f -`.  The rule is being inserted, but any 
rules which were already on that anchor are removed.  I was expecting 
that the new rule would be added to the rules on that anchor, not that 
the new rule would replace the rules on that anchor.

Is replacing the old anchor contents the expected behavior?  If not, 
does anyone have suggestions what the problem may be?

Thanks
Mark