Some definitions of interest.
bequal	Def x = y == (x = y  T)
	Thm* T:Type, x,y:T. (x = y)
label	Def t  ...$L == t
nat	Def  == {i:| 0i }
	Thm*   Type
	Thm*   S
not	Def A == A  False
	Thm* A:Prop. (A)  Prop

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