Nicola Willis fronts National's return-to-surplus and debt targets. (Image: Getty)The National Party has delivered the opening salvo declaring its Budget Responsibility Rules. Make your numbers add up.The Impossible Trinity (trilemma) is headed to the election.The conventional trilemma – as it is commonly referred to – is the view that it is impossible to have a fixed exchange rate, free flow of capital and independent monetary policy.You can have two, and most chose a floating currency not a fixed one.There is also an unconventional trilemma that involves the trade-offs political parties will make.The fiscal equ...