Sub-expression of an expression : part

part takes two arguments : an expression and an integer n.
part evaluate the expression and then returns the n-th sub-expression of this expression.
Input :

part(x^2+x+1,2)

Output :

x

Input :

part(x^2+(x+1)*(y-2)+2,2)

Output :

(x+1)*(y-2)

Input :

part((x+1)*(y-2)/2,2)

Output :

y-2