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