This trigger expression works as follows:
1. It can be triggered if the operations status is down.
2. `{$IFCONTROL:"{#IFNAME}"}=1` - a user can redefine the context macro to "0", marking this interface as not important. No new trigger will be fired if this interface is down.
3. `{TEMPLATE_NAME:METRIC.diff()}=1` - the trigger fires only if the operational status was up to "1" sometime before.

WARNING: if closed manually - it will not fire again on the next poll, because of `.diff`.