anchor.diff:  
     int get length => sequence.length;
   
  -  T operator[](int index) => sequence[index];
  +  E operator[](int index) => sequence[index];
   
     int indexOf(E value, [int start = 0]) {
       for (int i = start; i < sequence.length; i++) {
  
positive.code:  * It is intended for data structures where access by index is
   * the most efficient way to access the data.
   */
  abstract class Sequence<T> {
    /**
     * The limit of valid indices of the sequence.
     *
  
negative.code:    * Valid indices must be in the range [:0..length - 1:].
     * The lookup operator should be efficient.
     */
    E operator[](int index);
  }
  
  /**
  