old_contents: from csympy import Symbol, sin, cos, sqrt, Add, function_symbol
  
  def test_sin():
      x = Symbol("x")
      e = sin(x)
      assert e == sin(x)
      assert e != cos(x)
  
      assert sin(x).diff(x) == cos(x)
      assert cos(x).diff(x) == -sin(x)
  
      e = sqrt(x).diff(x).diff(x)
      f = sin(e)
      g = f.diff(x).diff(x)
      assert isinstance(g, Add)
  
  def test_f():
      x = Symbol("x")
      y = Symbol("y")
      f = function_symbol("f", x)
      g = function_symbol("g", x)
      assert f != g
  
      f = function_symbol("f", x)
      g = function_symbol("f", x)
      assert f == g
  
      f = function_symbol("f", x)
      assert f.diff(y) == 0
  
new_contents: from csympy import Symbol, sin, cos, sqrt, Add, function_symbol
  
  def test_sin():
      x = Symbol("x")
      e = sin(x)
      assert e == sin(x)
      assert e != cos(x)
  
      assert sin(x).diff(x) == cos(x)
      assert cos(x).diff(x) == -sin(x)
  
      e = sqrt(x).diff(x).diff(x)
      f = sin(e)
      g = f.diff(x).diff(x)
      assert isinstance(g, Add)
  
  def test_f():
      x = Symbol("x")
      y = Symbol("y")
      f = function_symbol("f", x)
      g = function_symbol("g", x)
      assert f != g
  
      f = function_symbol("f", x)
      g = function_symbol("f", x)
      assert f == g
  
      f = function_symbol("f", x)
      assert f.diff(y) == 0
  
  def test_derivative():
      x = Symbol("x")
      y = Symbol("y")
      f = function_symbol("f", x)
      assert f.diff(x) == function_symbol("f", x).diff(x)
      assert f.diff(x).diff(x) == function_symbol("f", x).diff(x).diff(x)
      assert f.diff(y) == 0
  
      g = function_symbol("f", y)
      assert g.diff(x) == 0
      assert g.diff(y) == function_symbol("f", y).diff(y)
      assert g.diff(y).diff(y) == function_symbol("f", y).diff(y).diff(y)
  
      assert f - function_symbol("f", x) == 0
  
subject: Add a test