DEF-SG. Singleton.
The singleton containing $a$, written as $\{a\}$, is the set $\{a,a\}$.
DEF-SG-SLF.
For any $X$,
\[X \neq \{X\}.\]
Proof.This is a consequence of Axiom of Regularlity.$\square$
DEF-SG. Singleton.
The singleton containing $a$, written as $\{a\}$, is the set $\{a,a\}$.
DEF-SG-SLF.
For any $X$,
\[X \neq \{X\}.\]
Proof.This is a consequence of Axiom of Regularlity.$\square$