Performs the set theoretic operation of union of this {@link MailboxACL}and toAdd {@link MailboxACL}. A schematic example: "user1:lr;user2:lrwt".union("user1:at;-$group1:lrwt") returns "user1:alrt;user2:lrwt;-$group1:lrwt". Implementations must return a new unmodifiable instance of {@link MailboxACL}. However, implementations may decide to return this or toAdd parameter value in case the result would be equal to the respective one of those.
@param toAdd
@return
@throws UnsupportedRightException
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