Problem 0.9

To prove A = B, where A and B are sets,
first prove A contains B and then prove that B contains A.
For more, see Section 1.3.1 of the HMU Chapter,
and for a non-trivial example see the back of
the Lecture 2 handout.

LEMMA:
If     C >= B  (set C contains set B),
then  AC >= AB (concatention of A and C contains concatenation of A and B)

[You may use this without proving, even though it is easy to prove.]