as B includes (or contains) A or A is included (or contained) in B. A k-subset' is a subset with k elements.

When quantified, is represented as

One can prove the statement by applying a proof technique known as the element argument:

Let sets A and B be given. To prove that

# suppose that a is a particular but arbitrarily chosen element of A
# show that a is an element of B.The validity of this technique can be seen as a consequence of universal generalization:

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