Suppose I have a Set $A$ with elements $P$:

$A = \{P\_1,P\_2,...,P\_n\}$

And another Set $B$, where the elements are Set $A$:

$B = \{A\_1,A\_2,...,A\_i\}$

How would I refer to a specific element $P$, as nested in $B$?

Would this be correct:
$P\_{B{\_i}n}$ ?

Thanks!