make spec less strict on overwriting memberships

standards/core/rln-contract.md

@@ -173,9 +173,11 @@ Membership registration is additionally subject to the following requirements:
 	- The new membership MAY overwrite one or multiple _Expired_ memberships.
 - If `r > R_{free}`:
 	- if `r > R_{free} + R_{expired}`, registration MUST fail;
-	- if `r <= R_{free} + R_{expired}`, the new membership MUST be registered by overwriting some _Expired_ memberships.
+	- if `r <= R_{free} + R_{expired}`, the new membership SHOULD be registered by overwriting some _Expired_ memberships.
 - The sender of the registration transaction MAY specify a list of _Expired_ memberships to be overwritten.
+	- If any of the memberships in the list are not _Expired_, the registration MUST fail.
 	- If the list is not provided, the contract MAY use any criteria to select _Expired_ memberships to overwrite (see Implementation Suggestions).
+	- If the list is not provided, the registration MAY fail even if the membership set contains _Expired_ membership that, if erased, would provide sufficient rate limit.
 - If a new membership A overwrites an _Expired_ membership B:
 	- membership B MUST become _ErasedAwaitsWithdrawal_;
 	- the current total rate limit MUST be decremented by the rate limit of membership B;