Boolean Chaining

fun leap(year):
  fun year-is-divisible-by(divisor):
    num-equal(num-modulo(year, divisor), 0)
  end

  year-is-divisible-by(4) and (not(year-is-divisible-by(100)) or year-is-divisible-by(400))
end

This approach uses a helper function year-is-divisible-by and the Boolean operators and and or to run each check (divisible by 4, not divisible by 100, and divisible by 400) in order, producing a single Boolean value at the end. The and operator returns true when both sides are true but false otherwise. The or operator returns false when both sides are false but true otherwise.

Both operators can short-circuit which means in some scenarios, the operators don't evaluate the expression to the right. For and, if the left side produces false, that value is returned immediately. For or, if the left side produces true, that value is returned immediately.

We can now test if a year is evenly divisible by 4, 100, and 400. All leap years are divisible by 4 but not by 100 unless they're also divisible by 100.