old_contents: from .regions import Boundary, MaterialRegion, Output
  from .acoustics import Acoustic1D, Acoustic2D, Acoustic3DAxi, AcousticMaterial
  from .gfx import Animator1D, Animator2D
  
new_contents: from .regions import Boundary, MaterialRegion, Output
  from .acoustics import Acoustic1D, Acoustic2D, Acoustic3DAxi, AcousticMaterial
  from .nonlinear_acoustics import NonlinearAcoustic1D
  from .gfx import Animator1D, Animator2D
  
subject: Update __init__.py