Extends Set's insert in swift for custom logic

I need to have custom logic in a Set that defines when a Hashable can be insert or not.

First I tried to solve this with a observer

var Tenants: Set<Tenant> = [] {
    willSet {
        // to the business logic here
        // ...

But in an observer i can not return an error. So I tried to extend Set to overwrite the insert method.

extension Set where Element == Tenant {
    @inlinable mutating func insert(_ newMember: Element) -> (inserted: Bool, memberAfterInsert: Element){

        // .... do my logic here ...

        return (true, newMember)
    }
}

That works so far and the method will be called. I can return true and if my logic did not pass even a false. Ok, but how do I add the Element into the Set? super.insert(). The return is correct, but the Set is empty. How to add the elements into the concrete set?

Implementation so far

/// Global set of known tenants
var Tenants: Set<Tenant> = [] {
    willSet {
        let newTenants = newValue.symmetricDifference(Tenants)
        guard let newTenant = newTenants.first else {
            Logging.main.error("Can not find tenant to add.")
            return
        }
        Logging.main.info("Will add new Tenant \(newTenant.name) [\(newTenant.ident)]")
    }
}

extension Set where Element == Tenant {

    @inlinable mutating func insert(_ newMember: Element) -> (inserted: Bool, memberAfterInsert: Element){
        print("Check to add...")
        // .... do my logic here ...

        // ok
        return (true, newMember)
    }

}

The result is:

Check to add...
error : Can not find tenant to add.
Check to add...
error : Can not find tenant to add.

>Solution :

This seems to work for "do my logic here"

    self = self.union([newMember])