This article is from the FAQ, by with numerous contributions by others.
Invariants are not an integrated part of the BETA language. However, it is
possible to create an invariant system as a framework or library, entirely
written in BETA. In the Mjolner System, an experimental invariant system is
available. The system offers class invariants as well as pre- and
post-conditions for operations. The invariant system also offers the ability
to control whether the invariants are checked or not, either on individual
object basis or system-wide.