code: def a2s(a):
      
      s = np.zeros((6,), 'f')  # make the a matrix
      for i in range(3):
          s[i] = a[i][i]
      s[3] = a[0][1]
      s[4] = a[1][2]
      s[5] = a[0][2]
      return s
docs: convert 3,3 a matrix to 6 element "s" list  (see Tauxe 1998)