input: Write a function to replace all occurrences of a given string with an alternate string in the input string. If the input string is empty, return the empty 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 Hello Hello", "Hello", "Hi") == "Hi Hi Hi"
  assert replace_string("No replacement", "replacement", "change") == "No change"
  assert replace_string("", "anything", "nothing") == ""
output: def replace_string(input_string, target_string, replacement_string):
      return input_string.replace(target_string, replacement_string)