Some definitions of interest.
absval	Def |i| == if 0i i else -i fi
	Thm* x:. |x|
iff	Def P  Q == (P  Q) & (P  Q)
	Thm* A,B:Prop. (A  B)  Prop
pm_equal	Def i =  j == i = j    i = -j
	Thm* a,b:. a =  b  Prop

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