code: def Vdiff(D1, D2):
      
      A = dir2cart([D1[0], D1[1], 1.])
      B = dir2cart([D2[0], D2[1], 1.])
      C = []
      for i in range(3):
          C.append(A[i] - B[i])
      return cart2dir(C)
docs: finds the vector difference between two directions D1,D2