A PVS specification of an invoicing system?

We present a PVS specification of an invoicing system. We use PVS’s features to structure the specification, to define types and operations on them. We put special emphasis on PVS’s capabilities to add information to a specification meant to be redundant, and to check that this information for consistency. We heavily use the possibility to define the type of some argument to a function depending on values of other arguments to the function, and to let the system automatically generate lemmas to be proved for checking consistency of function declaration and usage. PVS is found to be helpful because of the automatic support for doing proofs.