Say I have the set $A=\{\{1,2,\{3\}\}\}$ , and so the set cardinality of $A$ is $1$ from my understanding.
If $B = \{\{1,2,\{3\}\}, \{\{1,2,\{3\}\}\}$, then the set cardinality of B is $2$ too?
My question is
Suppose we have another set $C=\{ \emptyset \} \cdot A$,and another set $D=\{ \emptyset \} \cdot B $. Indeed, $ C \subseteq D$, but why isn't $C \in D$?
Because from my understanding the set $C$ can be written as $ \{(\emptyset , \{1,2,\{3\})\}$ and $D$ written as $ \{(\emptyset , \{1,2,\{3\}), (\emptyset , \{\{1,2,\{3\}\})\}$.
Sorry if my question isn't clear! Thanks for the help 🙂
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