code: def get_effective_ecs(self, strain, order=2):
          
          ec_sum = 0
          for n, ecs in enumerate(self[order-2:]):
              ec_sum += ecs.einsum_sequence([strain] * n) / factorial(n)
          return ec_sum
docs: Returns the effective elastic constants
          from the elastic tensor expansion.
  
          Args:
              strain (Strain or 3x3 array-like): strain condition
                  under which to calculate the effective constants
              order (int): order of the ecs to be returned