assert_eq!(ChangePrompt.execute(&mutBuiltinConfig::new(),vec!["1".to_string(),"2".to_string()]),Err(ShellError::ExecuteFailure("expected only one argument".to_string())))
assert_eq!(ChangePrompt.execute(&mutBuiltinConfig::new(),vec!["notexistent".to_string()]),Err(ShellError::ExecuteFailure("notexistent is not a valid prompt style".to_string())))