?. is ?: with the branches swapped - the yes branch comes second.

?.((gth 3 5) 10 20) is 10.

%: calls a gate with several arguments, gathering them into one sample.

%:(add 2 3) is 5.

irregular forms:

(gate a b c) is %:(gate a b c)

gth asks whether the first atom is strictly greater than the second.

(gth 5 3) is %.y.