input: Write a function to replace all occurrences of a given string with an alternate string.
  def replace_string(input_string, target_string, replacement_string):
      return input_string.replace(target_string, replacement_string)
  
  assert replace_string("Hello World!", "World", "Python")=="Hello Python!"
  assert replace_string("Hello World! World", "World", "Python")=="Hello Python! Python"
  assert replace_string("Hello World!", "o", "a")=="Hella Warld!"
  assert replace_string("Hello World!", "x", "y")=="Hello World!"
  assert replace_string("", "World", "Python")==""
output: def replace_string(input_string, target_string, replacement_string):
      return input_string.replace(target_string, replacement_string)