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by xcthulhu 5682 days ago
Check out the axioms of Peano Arithmetic: http://en.wikipedia.org/wiki/Peano_axioms#Arithmetic

If we use Dedekind's recursive definition of addition, and the definitions 1=S(0) and 2=S(S(0)), then:

S(0) + S(0) = S(S(0)+0) = S(S(0))

I know the computer proof assistant Isabelle/HOL, inspired by the Principia Mathematica, can prove simple theorems about arithmetic using a "rewrite" system similar to what I've done above.