Indeed, it does involve factorials. Using MariOne's notation (which is non-standard, but works on BBs):
( n k ) = n!/[(k!)(n-k)!]
( n k ) = n!/[(k!)(n-k)!]


)
what about *empty* bases? And aren't tectonic or fungal missiles considered as non-conventional? In such case, can't they be aimed just at terrain (i.e. an empty square)? And how would Flechette trigger in such case?
Comment