input: Write a function to reverse a given input string.
  
  def rev_string(s): 
      return s[::-1] 
  
  assert rev_string("Hello World!")=="!dlroW olleH"
  assert rev_string("Python")=="nohtyP"
  assert rev_string("12345")=="54321"
  assert rev_string("")==""
output: def rev_string(s): 
      return s[::-1]