/// Returns a subsequence, up to the specified maximum length, containing
  /// the initial elements of the collection.
  ///
  /// If the maximum length exceeds the number of elements in the collection,
  /// the result contains all the elements in the collection.
  ///
  /// ...
  /// - Parameter maxLength: The maximum number of elements to return.
  ///   `maxLength` must be greater than or equal to zero.
  /// - Returns: A subsequence starting at the beginning of this collection
  ///   with at most `maxLength` elements.
  @_inlineable
  public func prefix(_ maxLength: Int) -> SubSequence {
    _precondition(
      maxLength >= 0,
      "Can't take a prefix of negative length from a collection")
    let end = index(startIndex,
      offsetBy: maxLength, limitedBy: endIndex) ?? endIndex
    return self[startIndex..<end]
  }

extension String : StringProtocol, RangeReplaceableCollection {
  /// A type that represents the number of steps between two `String.Index`
  /// values, where one value is reachable from the other.
  ///
  /// In Swift, *reachability* refers to the ability to produce one value from
  /// the other through zero or more applications of `index(after:)`.
  public typealias IndexDistance = Int

  public typealias SubSequence = Substring

  // ...
}

func prefix(_ maxLength: Int) -> Substring