the European Council has put defence cooperation as a major project in its [post-Brexit vote] Bratislava and Rome declarations and moved forward with setting up a European Defence Fund and activating Permanent Structured Cooperation (a defence clause in the Lisbon Treaty). Freedom of movement Freedom of movement for workers is an integral part of EU policy and is a foundation of European Union ideals. In addition, the Schengen agreement removes the necessity for passports and visas for travel within the Schengen Area, thus allowing the free movement of people. This concept is designed to benefit the various member states' economy and society by allowing for business to thrive in Europe and also for the EU to be more culturally interconnected. (Visitors from outside the Area are subject to the usual passport and visa controls at the external border of any Area member state. Not all members of the EU are members of Schengen, and not all members of Schengen are members of the EU). Eurozone The relationship between euro and non-euro states has been on debate both during the UK's membership (as a large opt-out state) and in light of its withdrawal from the EU. The question is how Brexit might impact the balance of power between those in and out of the euro, namely avoiding a eurozone caucus out-voting non-euro states. The UK had called for the EU treaties to be amended to declare the EU to be a "multi-currency union", which sparked concerns that to do so might undermine the progress of euro adoption in remaining countries. Professor Simon Hix contended that Brexit would strengthen the Eurozone, which may well replace the single market as the EU's core and driving force. In the pre-referendum negotiation, David Cameron emphasised the importance (in his view) of keeping the Eurozone clearly distinct from the EU. Following a British withdrawal, such pressure may well dissipate. Speaking English in the European Parliament As a result of Brexit, there are now fewer native English speakers in the European Parliament and in workings groups. David McAllister, a German politician, stated that his former British colleagues are missed for their pragmatism, humour, and rhetoric. McAllister said that English is still the most common language spoken by members of the European Parliament despite it having 24 official languages. Economic impacts Trade with the UK After Brexit, the EU becomes UK's biggest trading partner, and the UK becomes