So… it seems this is impossible. ``` var x = muta...

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[benleggiero]

So… it seems this is impossible.

var x = mutableSetOf<Foo>()
var y = setOf<Foo>(Foo.instance)

x += y

I only get this:

Error:(41, 19) Kotlin: Assignment operators ambiguity: 
public operator fun <T> Set<Foo>.plus(elements: Iterable<Foo>): Set<Foo> defined in kotlin.collections
@InlineOnly public inline operator fun <T> MutableCollection<in Foo>.plusAssign(elements: Iterable<Foo>): Unit defined in kotlin.collections