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Donald Trump is the president-elect of the United States, even though political pundits predicted that he would not even win the Republican presidential nomination. In the end, more 59 million Americans voted for him. On inauguration day in January, many of Trump's voters will be looking forward - among other campaign promises - to the construction of a wall along the Mexican border. This means a physical barrier over 3,000 kilometres in length, on a border that one million people legally cross on a daily basis, producing half a trillion dollars in annual trade.