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We show

forall x in Z, y in Z: x+y = y+x.
Take arbitrary x in Z,y in Z. We have
x+y = (definition of +)
I(x +N y, x +N y) = (commutativity of +N)
I(y +N x, y +N x) = (definition of +)

More typical version.

Author: Wolfgang Schreiner
Last Modification: November 30, 1999

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