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Definitions
$x\text{ is even} := \exists y[y+y=x]$
$x\text{ is odd} := \exists y[y+y+1=x]$
Axioms
$\forall x[S(x) \ne 0]$
$\forall x \forall y[S(x)=S(y) \implies x=y]$
$\forall x[x+0=x]$
$\forall x \forall y[x+S(y)=S(x+y)]$
$[\varphi(0) \land [\forall x[\varphi(x) \implies \varphi(S(x))]]] ...