What are the necessary and sufficient conditions for a sofa and a fence

historically speaking, military dictators usually come into power

as the result of a military coup d'etat that was successful against  

whatever form of government existed before.  

sometimes the military dictator was put in place by winning a civil war.  

    occasionally military dictators conduct phony 'elections'  

in order to claim that they were elected by the people.

but there are never any serious candidates running against the dictator.

when a dictator becomes too old for governing his country,

he usually tries to arrange for one of his sons to take over power  

after he is dead and gone.  

when that is not possible, some secret decisions within the military

results in the next dictator taking power

—whether by means of an 'election' or another coup d'etat.  

    countries with little experience of other forms of government

often assume that a military strongman is their natural leader.  

military dictators want to stay in power for as long as possible.  

often the only way they leave office is feet first,

that is, they must die or be killed while still in power  

before the next form of government will take over.

explanation: