Is the set 1,2,3,4,5\\{1, 2, \\{3, 4\\}, 5\\}1,2,3,4,5 a subset of 1,2,3,4,5\\{1, 2, 3, 4, 5\\}1,2,3,4,5?
Yes
No, because 3,4\\{3, 4\\}3,4 is an element, not 3 and 4 separately
Yes, if order doesn't matter
Only if we flatten the set