Some definitions of interest.
absval	Def |i| == if 0i i else -i fi
	Thm* x:. |x|
gcd	Def gcd(a;b) == if b=0 a else gcd(b;a rem b) fi  (recursive)
	Thm* a,b:. gcd(a;b)
not	Def A == A  False
	Thm* A:Prop. (A)  Prop

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