/// The total number of single-hop index steps made in our test collections.
var totalSteps = 0
/// A collection that tracks the number of single-hop index steps executed.
struct CountSteps0 : RandomAccessCollection {
 init(count: Int) { self.count = count }
 var count: Int
var startIndex: Int { 0 }
 var endIndex: Int { count }
 func index(after i: Int) -> Int { 
 totalSteps += 1
 return i + 1
 }
 func index(before i: Int) -> Int { 
 totalSteps += 1
 return i - 1
 }
 subscript(i: Int) -> Range<Int> { return 0..<9 }
}
extension Collection {
 func midPoint() -> Index { 
 index(startIndex, offsetBy: count / 2, limitedBy: endIndex)!
 }
}
let x0 = CountSteps0(count: 900)
_ = x0.midPoint()
print("steps:", totalSteps) // 0
_ = x0.joined().midPoint()
print("steps:", totalSteps) // 1350