input: Write a Python function that takes two arguments, a string (str) and a single replacement character (replacement), and returns the string with all occurrences of the replacement character replaced by a specified replacement character. If the string does not contain the replacement character, the function should return the original string.
  
  
  def replace_all_occurrences(str, char, replacement):
      return str.replace(char, replacement)
  
  assert replace_all_occurrences("Hello World!", 'l', 'X') == "HeXXo WorXd!"
  assert replace_all_occurrences("Python is fun", 'n', 'N') == "PythoN is fuN"
  assert replace_all_occurrences("No changes here", 'z', 'X') == "No changes here"
  assert replace_all_occurrences("", 'a', 'b') == ""
  assert replace_all_occurrences("Single character", 'c', 's') == "Single sharasater"
  
output: def replace_all_occurrences(str, char, replacement):
      return str.replace(char, replacement)