A FieldRule is a list of cases that will be evaluated on every insert or update.
A RuleCase represents one case for a FieldRule, with a test, a list of assignments for
an insert operation, and a list of assignments for an update operation.
The first case whose test is true will be executed.
If none are true and there is a default case, that case will be executed.
If the RuleCase used as default includes a test, the test will be ignored.