I am working on a problem with the following set:
$S = \{\varnothing,\{\varnothing\}\}$
My solution was: $P(S) = \{\varnothing, \{\varnothing\},\{ \varnothing , \{ \varnothing \}\}\}$, but the solution on the textbook shows: $P(S) = \{ \varnothing , \{ \varnothing \}, \{\{\varnothing\}\}, \{ \varnothing , \{ \varnothing \}\}$
Where does the $\{\{\varnothing\}\}$ come from?
Thanks