new_contents: from pymc3 import Model, Normal, Metropolis, MvNormal
  from numpy import ma
  
  def test_missing():
      data = ma.masked_values([1,2,-1,4,-1], value=-1)
      with Model() as model:
          x = Normal('x', 1, 1)
          y = Normal('y', x, 1, observed=data)
  
      y_missing, = model.missing_values 
      assert y_missing.tag.test_value.shape == (2,)
  
      model.logp(model.test_point)
  
  
  
subject: test missing values