The Quiz:-Set command works inside an initialization or grading procedure in conjunction with the Quiz command.

All variables used by Set must be of the form , where the name has a leading dollar sign, and the variable name is surrounded by left single quotes.

The Quiz command is a module, but not a package, so it should not be loaded via with(Quiz).  However, since Set is only relevant inside a procedure, it is recommended that the first line of the procedure be the statement, uses Grading:-Quiz; so the short-form of the command name can be used.

The Grading[Quiz][Set] command was introduced in Maple 18.

For more information on Maple 18 changes, see Updates in Maple 18.