The treaty has a safeguard clause that any country can invoke if it wishes to retain or impose border controls.

ECONOMIST: Those fuzzy frontiers

Its biggest weapon is a safeguard clause under which Bulgarian court decisions would no longer be automatically recognised in other member states.

ECONOMIST: Bulgarian corruption

Then on Wednesday, Switzerland, not a member of the EU but part of its passport-free travel zone, invoked a special safeguard clause that allows it to cap the number of workers from 28 European countries.

WSJ: Europeans Stay Rooted, Despite Economy