section_path: Singleton sets
section_name: Singleton sets
paragraph_text: Singleton sets
  
  A singleton set is a set with exactly one element; such a set may also be called a unit set. Any such set can be written as { x }, where x is the element.The set { x } and the element x mean different things; Halmos draws the analogy that a box containing a hat is not the same as the hat.