code: def _len_lcs(x, y):
      
      table = _lcs(x, y)
      n, m = _get_index_of_lcs(x, y)
      return table[n, m]
docs: Returns the length of the Longest Common Subsequence between sequences x
      and y.
      Source: http://www.algorithmist.com/index.php/Longest_Common_Subsequence
  
      :param x: sequence of words
      :param y: sequence of words
      :returns integer: Length of LCS between x and y