If a parameter is required but provides a default value, use the default value lower bound as the parameter lower bound

[abstratt]

For instance, in the signature below, clients should be able to omit a value for date, and the implementation of newCharge should still be able to rely on it not being null.

    static operation newCharge(taxi : Taxi, payer : Driver, date : Date := { Date#today() });

We either need to relax the verification on the operation call site, or allow the modeler the declare the parameter as optional and still consider it to be required in the body of the operation implementation.