code: def calculate_stress(self, strain):
          
          strain = np.array(strain)
          if strain.shape == (6,):
              strain = Strain.from_voigt(strain)
          assert strain.shape == (3, 3), "Strain must be 3x3 or voigt-notation"
          stress_matrix = self.einsum_sequence([strain]*(self.order - 1)) \
                  / factorial(self.order - 1)
          return Stress(stress_matrix)
docs: Calculate's a given elastic tensor's contribution to the
          stress using Einstein summation
  
          Args:
              strain (3x3 array-like): matrix corresponding to strain