Invader board only handles three stacked cards.

[pyrrhicPachyderm]

The invader board only handles three cards in a slot; more won't advance properly. It's not a problem right now, as that's the most that can end up in a slot pre-NI (one extra from Russia, and one from Immigration Slows or Explorers Are Reluctant). However, with Surges of Colonisation coming in NI, this will need to handle up to five cards at a time.