`alreadyfiltered` mechanism was introduced (under a different name,
with less abstraction) by [[intrigeri]] in commit 1e874b3f:
-```
-po plugin[filter]: avoid converting more than once per destfile
+ po plugin[filter]: avoid converting more than once per destfile
-Only the first filter function call on a given {page,destpage} must convert it
-from the PO file, subsequent calls must leave the passed $content unmodified.
+ Only the first filter function call on a given {page,destpage} must convert it
+ from the PO file, subsequent calls must leave the passed $content unmodified.
-Else, preprocessing loops are the rule.
-```
+ Else, preprocessing loops are the rule.
I don't understand this. Under what circumstances would we pass content
through the filter hooks, and then pass it back through the same filter