Full Idea
{a} is the 'singleton' set of a, not to be confused with the object a itself.
Gist of Idea
{a} is the 'singleton' set of a (not the object a itself)
Source
Graham Priest (Intro to Non-Classical Logic (1st ed) [2001], 0.1.4)
Book Reference
Priest,Graham: 'Introduction to Non-Classical Logic' [CUP 2001], p.-7