A subset 
of the real numbers is said to be an 
set provided 
is the countable union of closed sets. The name 
comes from French: The F stands for fermé,
meaning "closed," while the 
stands for somme, meaning "sum."
This definition is dual to that of a Gdelta set in the sense that 
is an 
set if and only if its complement 
is a 
.
Note that both 
and 
sets are fundamental to measure
theory. In particular, both types of sets are Borel
sets and are considered to be the second level of the so-called Borel
hierarchy.
