input: Write a function to replace all occurrences of a character in a given string with a provided replacement character. The function should take three parameters: the original string, the character to be replaced, and the replacement character. If the character to be replaced is not found in the string, the function should return the original string unchanged.
  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("Hello World!", 'o', 'Y')=='HellY WYrld!'
  assert replace_all_occurrences("Hello World!", 'Z', 'A')=='Hello World!'
  assert replace_all_occurrences("", 'a', 'b')==''
  assert replace_all_occurrences("abcabc", 'a', 'X')=='XbcXbc'
output: def replace_all_occurrences(str, char, replacement):
    return str.replace(char, replacement)