How to union nested Sets / A single set from nested sets?

[mviljamaa]

I'm forming sets by set.adding to sets and this leads to sets such as:

Set([ImmutableSet(['a', ImmutableSet(['a'])]), ImmutableSet(['b', 
'c'])])

Is there way union these to a single set, i.e. get

Set(['a', 'b', 'c'])

?