input: Write a function to replace all occurrences of a specified character in a given string with the provided replacement character.
  
  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', 'Z')=="PythoZ is fuZ!"
  assert replace_all_occurrences("Coding is great", 'g', 'X')=="Coding is Xreat"
  assert replace_all_occurrences("", 'a', 'b')==""
  assert replace_all_occurrences("Hello", 'z', 'x')=="Hello"
output: def replace_all_occurrences(str, char, replacement):
    return str.replace(char, replacement)