Variables can be used in both test and assignment expressions in the FieldRules. If there is a non-trivial expression
that will be used more than once in the evaluation of FieldRules, then use a Variable to save the value of that
expression. Note that all Variables will be evaluated for every insert and update of every value.
Variables can also be the target of assignments in RuleCases, if there are expressions that only need to
be evaluated for certain cases.