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("Python is fun!", "Python", "Java")=="Java is fun!"
  assert replace_string("Coding is cool.", "Coding", "Programming")=="Programming is cool."
  assert replace_string("This is a test.", "This", "That")=="That is a test."
  assert replace_string("Empty case", "Empty", "")==" case"
output: def replace_string(input_string, target_string, replacement_string):
      return input_string.replace(target_string, replacement_string)