+Note [Where rules are found]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+The rules for an Id come from two places:
+ (a) the ones it is born with, stored inside the Id iself (idCoreRules fn),
+ (b) rules added in other modules, stored in the global RuleBase (imp_rules)
+
+It's tempting to think that
+ - LocalIds have only (a)
+ - non-LocalIds have only (b)
+
+but that isn't quite right:
+
+ - PrimOps and ClassOps are born with a bunch of rules inside the Id,
+ even when they are imported
+
+ - The rules in PrelRules.builtinRules should be active even
+ in the module defining the Id (when it's a LocalId), but
+ the rules are kept in the global RuleBase
+