SPECADT05: CafeObj & Error Values

Dear course participants,

I forgot to discuss in class how to deal in CafeObj with
the error values demanded by the "FileSystem" specification.

I suggest to add an error constant to each type that is the domain
of a function with a possible error result and return this constant
in case of errors. If you do not equate this constant with
any valid value of that type (I hope you do not

then it is clear
that this value is different from any valid value and can be
used in checking for errors.

Best regards, WS