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**A basic question: What does "closed form" mean?**

"The point here is that [itex]\sigma[/itex] algebras are difficult but [tex]\pi[/tex] systems are easy: one can often write down in closed form the general element of a [tex]\pi[/tex] system while the general element event of [itex]\mathbf B \mathbf[/itex] is impossibly complicated" - From the "Diffusions" text by Rogers and Williams.

What does closed form mean?