Universal 21: SA with non-possessor case ⇒ SA with possessor case
Original
In almost all languages if the internal case involved in Suffixaufnahme is a case other than that of the possessor, the case of the possessor may also be involved in Suffixaufnahme.
Standardized
IF the internal case involved in Suffixaufnahme is a case other than that of the possessor, THEN the case of the possessor (typically genitive) is almost always also involved in Suffixaufnahme.