4.30 How to find all symbols that represent variables in an expression?

Given an expression such as \(a+\sin (x) + \pi + y+ f(r) \) how to find all symbols in it, which will be \(a,x,y,r\)?

One way

expr:=a+sin(x) + Pi + y+ f(r); 
indets(expr,And(symbol,Not(constant)));
 

Another is

expr:=a+sin(x) + Pi + y+ f(r); 
indets(expr,assignable(name));
 

Both give {a, r, x, y}