set notation for multisets is broken: uses ∪ instead of ⊎

The notation {[ x1 ; x2 ]} expands to {[ x1 ]} ∪ {[ x2 ]} instead of {[ x1 ]} ⊎ {[ x2 ]}. This is wrong.

This was caused by !57 (merged)

I don't see an obvious way of fixing this problem, since the notation {[ x1; ..; xn ]} is generic for any singleton and union instance.