Sigh. The current design of Junctions is quite extensible *because*
it is based on a real type. You can easily write operators that
work only on Junctions, just as you can easily write operators that
are transparent to Junctions or autothread on Junctions by declaring
their arguments to take Object or Any, respectively.
That being said, the reason Junctions are there in Perl 6 is primarily
*linguistic* rather than mathematical; Perl 6 has a Set type because
we've had this argument already. Anyone who extends Junctions to
try to do some kind of set theory is on the road to Perdition, or at
least the road to Brain Pretzels Without End, amen.
In my opinion. :-)