Kara walks the slalom in an endless loop around the trees. He alternates left and right turns as follows:

while true
  while kara.treeLeft and not kara.treeRight
    quarterTurn(kara) { kara.turnLeft }
  end

  quarterTurn(kara) { kara.turnRight }

  while not kara.treeLeft and kara.treeRight
    quarterTurn(kara) { kara.turnRight }
  end

  quarterTurn(kara) { kara.turnLeft }
end