Element
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An element, also called a member, is an object contained within a set or class.
means set 
contains the elements 1, 2, 3 and 4.
To show that an element is contained within a set, the 
symbol is used. The opposite of is 
, which means the element is not contained within the set.
Sets as Elements
Elements can also be sets. For example, 
. The elements of 
are 
, 
, and 
.
