put all the details in the interface/definition module/package and trust the users to use the interface

make the type opaque (it is really a pointer, and the record to which it points is in the implementation)

state pre and post conditions even if they cannot be enforced

put all the details in the interface/definition module/package, but mark the fields as private

not to use libraries at all and require programmers to maintain their own code entirely contained in their programs