# Closure law in definition of groups [duplicate]

Possible Duplicate:
Why is closure omitted in some group definitions?

I have just started studying about groups, so pardon me if this is a lame question

while i was reading the definition. the author states that

'strictly speaking the closure law is not necessary since a binary operation on a set necessarily satisfies it'

can anyone elaborate on this statement?i am not sure i quite understand it

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