old_contents: import pytest
  import pandas as pd
  
  from tardis.io.decay import IsotopeAbundances
  
  @pytest.fixture
  def simple_abundance_model():
      index = pd.MultiIndex.from_tuples([(28, 56)],
                                        names=['atomic_number', 'mass_number'])
      return IsotopeAbundances([[1.0, 1.0]], index=index)
  
  
  def test_simple_decay(simple_abundance_model):
      1/0
  
new_contents: import pytest
  import pandas as pd
  
  from tardis.io.decay import IsotopeAbundances
  from numpy.testing import assert_almost_equal
  
  @pytest.fixture
  def simple_abundance_model():
      index = pd.MultiIndex.from_tuples([(28, 56)],
                                        names=['atomic_number', 'mass_number'])
      return IsotopeAbundances([[1.0, 1.0]], index=index)
  
  def test_simple_decay(simple_abundance_model):
      decayed_abundance = simple_abundance_model.decay(100)
      assert_almost_equal(decayed_abundance.ix[26, 56][0], 0.55752)
      assert_almost_equal(decayed_abundance.ix[26, 56][1], 0.55752)
      assert_almost_equal(decayed_abundance.ix[27, 56][0], 0.4423791)
      assert_almost_equal(decayed_abundance.ix[27, 56][1], 0.4423791)
      assert_almost_equal(decayed_abundance.ix[28, 56][0], 1.1086e-05)
      assert_almost_equal(decayed_abundance.ix[28, 56][1], 1.1086e-05)
  
subject: Add unit test for decay method