old_contents: from devito import SubsampledDimension, Grid, TimeFunction, Eq, Operator
  from devito.tools import pprint
  
  
  def test_subsampled_dimension():
      nt = 10
      grid = Grid(shape=(11, 11))
      x, y = grid.dimensions
      time = grid.time_dim
      t = grid.stepping_dim
      time_subsampled = SubsampledDimension('t_sub', parent=time, factor=4)
      u = TimeFunction(name='u', grid=grid)
      u2 = TimeFunction(name='u2', grid=grid, save=nt)
      assert(t in u.indices)
      u_s = TimeFunction(name='u_s', grid=grid, time_dim=time_subsampled)
      assert(time_subsampled in u_s.indices)
      fwd_eqn = Eq(u.indexed[t+1, x, y], u.indexed[t, x, y] + 1.)
      fwd_eqn_2 = Eq(u2.indexed[time+1, x, y], u2.indexed[time, x, y] + 1.)
      save_eqn = Eq(u_s, u)
      #fwd_op = Operator([fwd_eqn])
      fwd_op = Operator([fwd_eqn, fwd_eqn_2, save_eqn])
      pprint(fwd_op)
      print(fwd_op)
  
new_contents: from devito import SubsampledDimension, Grid, TimeFunction, Eq, Operator
  from devito.tools import pprint
  
  
  def test_subsampled_dimension():
      nt = 10
      grid = Grid(shape=(11, 11))
      x, y = grid.dimensions
      time = grid.time_dim
      t = grid.stepping_dim
      time_subsampled = SubsampledDimension('t_sub', parent=time, factor=4)
      u = TimeFunction(name='u', grid=grid)
      u2 = TimeFunction(name='u2', grid=grid, save=nt)
      assert(t in u.indices)
      u_s = TimeFunction(name='u_s', grid=grid, time_dim=time_subsampled)
      assert(time_subsampled in u_s.indices)
      fwd_eqn = Eq(u.indexed[t+1, x, y], u.indexed[t, x, y] + 1.)
      fwd_eqn_2 = Eq(u2.indexed[time+1, x, y], u2.indexed[time, x, y] + 1.)
      save_eqn = Eq(u_s, u)
      #fwd_op = Operator([fwd_eqn])
      fwd_op = Operator([fwd_eqn, fwd_eqn_2, save_eqn])
      #pprint(fwd_op)
      #print(fwd_op)
  
subject: Change from is_Stepping to is_Derived wherever appropriate