Tunisia, which had two presidents since becoming independent in 1956, has now had a series of three people claiming the post in less than 24 hours. What is going on?

Yesterday, President Zine El Abidine Ben Ali left the country a bit too hurriedly to either resign or leave instructions. So the prime minister stepped in front of television cameras and claimed he would temporarily assume presidential duties. He cited Article 56 of the Tunisian constitution. Its provisions allow the prime minister to assume the role of acting president when the president is temporarily unable to serve and issues a decree deputizing the premier.
Politically Ben Ali may have been dead, but constitutionally he was just on vacation. Opposition leaders cried foul.  In legalistic terms, where was the required decree? And in practical terms, this was a partial step at best. The regime stood as it had before, minus its head. There was no time limit to the "temporary" measure, and no new elections were scheduled.
So today,under continuing pressure, the scrambling and panicking regime moved down to Article 57, covering vacancy in the presidency. The Constitutional Council can declare the president incompetent to serve, allowing the speaker of the parliament to take the post as long as new elections are held within 60 days.
Do constitutional provisions really matter in a place like this? Can't rulers just do what they like? Is the opposition likely to accept a set of rules tailor-made for an authoritarian regime? The answer to the first two questions is yes. Constitutions can matter even in a place like Tunisia. The answer to the third question is more difficult -- use of the constitution would be a mixed bag for the opposition.
In normal times, constitutional provisions matter in a place like Tunisia because they give the rulers the tools to do whatever they want. Even authoritarian regimes need clear chains of command and authoritative structures just as much as liberal democratic ones. So they almost always issue constitutions and generally follow them -- it is just that the constitutional provisions are neither liberal nor democratic (and what seemingly liberal and democratic provisions exist are qualified out of meaningful existence). Of course, they do not want to have that constitutional language that will restrict them in any way. If that happens, they will show fewer scruples. But that is why Article 56 was so handy. By relying on it, Tunisia's remaining rulers seemed to be saying: "We are still in charge and we can still do what we want." That was why the step was less than satisfying.
But that also leads us to understand why constitutions can matter even in times of crisis. Today in Tunisia the constitution provides the only framework for the interim regime and the opposition to negotiate. Of course, revolutions in which constitutions are completely forgotten certainly occur. But not all regime changes happen that way -- sometimes they can be negotiated through existing constitutional mechanisms. That often happened in 1989 when communism fell in Eastern Europe, for instance. And in a case like Tunisia, in which decades of political repression have led to an opposition with weak leadership that has little ability to develop a detailed and unifying program, following the constitution may give it the breathing spell it needs while allowing state institutions to continue to function in the vacuum.
But Article 57 -- if that is what is used -- is a very mixed blessing for the opposition. The problems start when you read the fine print. The presidential elections have to be held according to the current constitutional provisions, and those allow only the Potemkin parliament (and a few other officials) the ability to nominate candidates. And while the acting president is serving, no constitutional amendments are allowed. In other words, invocation of Article 57 kicks into gear a process that was carefully designed for Ben Ali. It is designed for a figure handpicked by current top leaders, not for a truly open election.