[xyzzyz]

The fact that definition can be evaluated many times is not really relevant here. What is important is how one specifies language's semantic -- for instance, Common Lisp: The Language, 2nd edition book (I don't own ANSI standard) says:

When the function represented by the lambda expression is applied to arguments, the arguments and parameters are processed in order from left to right. (...) If optional parameters are specified, then each one is processed as follows. If any unprocessed arguments remain, then the parameter variable var is bound to the next remaining arguments, just as for required parameter. If no arguments remain, however, then the initform part of the parameter specifier is evaluated, and the parameter variable is bound to the resulting value (...).

The CLTL2 specifies that the form representing the default value of optional parameter shall be evaluated every time the parameter is not provided.