code: function(xs, ys, matrix){
    var lcs = [];
    for(var i = xs.size, j = ys.size; i !== 0 && j !== 0;){
      if (matrix[i][j] === matrix[i-1][j]){ i--; }
      else if (matrix[i][j] === matrix[i][j-1]){ j--; }
      else{
        if(Immutable.is(xs.get(i-1), ys.get(j-1))){
          lcs.push(xs.get(i-1));
          i--;
          j--;
        }
      }
    }
    return lcs.reverse();
  }
docs: Extracts a LCS from matrix M
  @param xs Indexed Sequence 1
  @param ys Indexed Sequence 2
  @param matrix LCS Matrix
  @returns {Array.<T>} Longest Common Subsequence