A derived group containing more than the commutators [duplicate]

Possible Duplicate:
Commutator subgroup does not consist only of commutators?

I am looking for an simple example (i.e. for undergraduate students) of group whose commutators $xyx^{-1}y^{-1}$ do not form a subgroup.

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marked as duplicate by HJRW, Mark Sapir, Chris Godsil, Tony Huynh, Richard KentJan 11 '12 at 12:54

This question was marked as an exact duplicate of an existing question.

I'm voting to close this as a duplicate of this question: mathoverflow.net/questions/44269/… . – HJRW Jan 11 '12 at 12:40