func Search(n int, f func(int) bool) int {
                // Define f(-1) == false and f(n) == true.
                // Invariant: f(i-1) == false, f(j) == true.
                i, j := 0, n
                for i < j {
                        h := i + (j-i)/2 // avoid overflow when computing h
                        // i ≤ h < j
                        if !f(h) {
                                i = h + 1 // preserves f(i-1) == false
                        } else {
                                j = h // preserves f(j) == true
                        }
                }
                // i == j, f(i-1) == false, and f(j) (= f(i)) == true  =>  answer is i.
                return i
        }