   /// -
   /// - If `head` is normal, then `abs(tail) < head.ulp`.
   ///   Assuming IEEE 754 default rounding, `abs(tail) <= head.ulp/2`.
+  ///
+  /// ["fastTwoSum"]:  https://en.wikipedia.org/wiki/2Sum
   @_transparent
   public static func sum<T:Real>(large a: T, small b: T) -> (head: T, tail: T) {
     assert(!(b.magnitude > a.magnitude))

    let tail = a - head + b
    return (head, tail)
  }
}

  /// never underflow. However, it may not be exactly representable when
  /// `a` and `b` differ widely in magnitude.
  ///
  /// This operation is sometimes called ["fastTwoSum"].
  ///
  /// - Parameters:
  ///   - a: The summand with larger magnitude.
