What are pre-conditions, post-conditions, and invariants?

 A pre-condition test (Eiffel Require assertion) guarantees
 that a subprogram will not be entered unless the assertion is
 satisfied.  It can, in some environments, be applied at the
 class/type level, but is more often at the subprogram level.
 An example of this assertion is the "barrier" on a protected
 entry.  The barrier is a lot like a pre-condition since it also
 evaluates a boolean expression before we can begin execution of
 the routine.  The barrier is not precisely identical to a pre-
 condition, but it should give you the general idea.

 A post-condition (Eiffel Ensure) guarantees that a certain set
 of circumstances will be true before exiting a subprogram.  If
 the post-condition fails, an exception may be raised.  

 An invariant is much as it might seem to be from its name.  During
 the life of the entity given an invariant condition, the invariant
 must be true.  

 The Require, Ensure, and Invariant conditions may be applied to 
 a subprogram, a type, or a even some variable.  The designer of
 Eiffel (who did not invent these ideas but incorporated them into
 the design of his language), calls this "design by contract."  It
 is important to understand that this is a run-time contract.