I want to know the answer to the following question:
What did Swiss voters agree to join on June 5, 2005?
All the background I'm provided with is that "On 5 June 2005, Swiss voters agreed by a 55% majority to join the Schengen treaty, a result that was regarded by EU commentators as a sign of support by Switzerland, a country that is traditionally perceived as independent and reluctant to enter supranational bodies.". Is that enough to answer the question?