Definitions for "class invariant"
a boolean expression that must be true after a constructor has created an object of the class, and that must be true after an operation has modified an object of the class
a condition that defines all valid states for an object
a condition that should always be true about a class
an example of a crosscutting concern
Keywords:  devised, whenever, simple, rule, hold
a simple rule, devised by the designer of the class, that must hold whenever a member function is called
Keywords:  pre, condition, post, kind, entire
a condition that applies to an entire class
a kind of pre- and post-condition