new_contents: import numpy as np
  import anndata as ad
  import os
  
  X = np.array([[1, 2, 3],
                [4, 5, 6],
                [7, 8, 9]])
  L = np.array([[10, 11, 12],
                [13, 14, 15],
                [16, 17, 18]])
  
  def test_creation():
      adata = ad.AnnData(X=X, layers_X={'L':L.copy()})
  
      assert list(adata.layers_X.keys()) == ['L']
      assert (adata.layers_X['L'] == L).all()
  
  def test_views():
      adata = ad.AnnData(X=X, layers_X={'L':L.copy()})
      adata_view = adata[1:, 1:]
  
      assert adata_view.layers_X.isview
      assert adata_view.layers_X._adata_ref == adata
  
      assert adata_view.layers_X.keys() == adata.layers_X.keys()
      assert (adata_view.layers_X['L'] == adata.layers_X['L'][1:, 1:]).all()
  
      adata.layers_X['S'] = X
  
      assert adata_view.layers_X.keys() == adata.layers_X.keys()
      assert (adata_view.layers_X['S'] == adata.layers_X['S'][1:, 1:]).all()
  
      adata_view.layers_X['T'] = X[1:, 1:]
  
      assert not adata_view.layers_X.isview
      assert not adata_view.isview
  
  def test_readwrite():
      adata = ad.AnnData(X=X, layers_X={'L':L.copy()})
      adata.write('test.h5ad')
  
      adata_read = ad.read_h5ad('test.h5ad')
  
      assert adata.layers_X.keys() == adata_read.layers_X.keys()
      assert (adata.layers_X['L'] == adata_read.layers_X['L']).all()
  
      os.remove('test.h5ad')
  
  def test_backed():
      #backed mode for layers isn't implemented, layers stay in memory
      pass
  
subject: add some tests for layers