The European project has always been first and foremost a political creature.  It was a political decision to establish the European Coal and Steel Community which led eventually to the EU and even the establishment of the Euro was driven by the political will to force the continental economies closer together rather than await organic economic harmonisation.

That process has often involved, for expediency, putting off until tomorrow decisions that could not be settled today and that has left a lot of constitutional chickens flapping around awaiting their chance to come home to roost.

Which is how we now have a situation where questions are being asked to clarify who exactly has the constitutional capacity to trigger Article 50.

Given the complexity of the issues Brian Taylor has done an great job of teasing apart the various strands.  To try to summarise further his excellent summary: The Scotland Act reserves to Westminster the decisions on international relations but devolves to Holyrood decisions on EU law.  So under which particular hat does Article 50 sit? 

By convention Westminster does not interfere in devolved areas without the consent of Holyrood so it may be that Scotland's consent IS needed to trigger Article 50, at least as far as Scotland is concerned - but that is a circular argument.  We have to decide first whether this decision is devolved or not.

And more acutely, it is a principle of Parliamentary sovereignty that one Parliament cannot bind a successor so in theory (but almost certainly only in theory) Westminster could legislate away any veto Scotland thought it might have.

Somewhere in all of this Brussels may have the difficult job of refereeing between the possible positions of a UK Prime Minister issuing an Article 50 declaration and a Scottish parliament arguing that such a move would not bind Scotland.

At least we can say that all of the politicians involved wanted the job, although whether this was what they were expecting is another question.