AddOriginalElement(originalIndex, modifiedIndex) {
        // The 'true' start index is the smallest of the ones we've seen
        this.m_originalStart = Math.min(this.m_originalStart, originalIndex);
        this.m_modifiedStart = Math.min(this.m_modifiedStart, modifiedIndex);
        this.m_originalCount++;
    }
Adds the original element at the given position to the elements
affected by the current change. The modified index gives context
to the change position with respect to the original sequence.
@param originalIndex The index of the original element to add.
@param modifiedIndex The index of the modified element that provides corresponding position in the modified sequence.