public interface AppendUndoModel
type AppendUndoModel is modeled by (input: string of character, output: string of string of character)
default: ensures this = ("", <>)
String
input()
Stack<String>
output()
void
setInput(String input)
String input()
input = this.input
void setInput(String input)
input
this.input = input
Stack<String> output()
output
output = this.output