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"If he shall be Mr. Hyde, I shall be Mr. Seek" is an example of this.
What is a pun?
A pun is a form of word play in which a word suggests two or more meanings. (Example: "Doctors tell us there are close to 1 million patients who are overweight. This is just a round figure, of course.") <-- In this example, "round" could have two meanings, one of which is comical.
Mr. Utterson says this in chapter 2 (Search for Mr. Hyde), providing comic relief but also elucidating the symbolic meaning of Mr. Hyde's name (symbolic for the "hidden" elements of man)