How do I effectively target a specific key-modifier combination in an XKB interpret statement?

I want to lock a certain modifier (Mod3) when pressing Control + Caps_Lock.

xkb_compatibility "mod3"
{
	// set Control bit on Control_L
	interpret Control_L {
		action = SetMods(modifiers=Control);
	};
	// try locking Mod3 …
	interpret Caps_Lock+Exactly(Control) {
		action = LockMods(modifiers=Mod3);
	};
};

Using xev -event keyboard I never saw the Mod3 bit flipped, even though the Control bit was. I also tried using just a regular a instead of Caps_Lock, which didn’t change anything. Further, I tried using AnyOf(Control) instead of Exactly(Control). --- What am I missing? Could anything else in my XKB map interfere with the above attempt? Or is my assumption about how the interpret statement works just wrong?