The Retrograde Analysis Corner
Patrol chess is a fairy condition in which no unit (King
included) may capture unless it is covered (that is, observed)
by a friendly unit. A non-covered unit does not give check (because
it can't capture the enemy King).
Here the "patrol" is the couple made by capturing/checking unit together
with its friendly observer. The Patrol condition was invented in 1975
by F. H. von Meyenfeldt.
Here is an example (where one has to decide whether Black's O-O-O
7+10. Shortest possible mate? (Patrol chess)