input: Write a function to replace all occurrences of a given string with an alternate string. If the given string is empty then return the original string. 
  assert replace_string("Hello World!", "World", "Python")=="Hello Python!"
  assert replace_string("The quick brown fox jumps over the lazy dog.", "the", "a")=="The quick brown fox jumps over a lazy dog."
  assert replace_string("Apple, apple, pineapple", "apple", "orange")=="Apple, orange, pineapple"
  assert replace_string("", "anything", "nothing")==""
  assert replace_string("Example String", "String", "")=="Example "
output: def replace_string(original, target, replacement):
      if target == "":
          return original
      return original.replace(target, replacement)