input: Write a python function to register cell magics "if_" and "for_" for IPython.
  assert if_('x > 10', 'print("Greater than 10")')=='if x > 10:\n    print("Greater than 10")'
  assert for_('i in range(5)', 'print(i)')=='for i in range(5):\n    print(i)'
output: def if_(condition, code):
      return f'if {condition}:\n    {code}'
  
  def for_(loop, code):
      return f'for {loop}:\n    {code}'