Some definitions of interest.
preserved_by	Def R preserves P == x,y:T. P(x)  (x R y)  P(y)
	Thm* T:Type, P:(TProp), R:(TTProp). R preserves P  Prop
rel_or	Def (R1  R2)(x,y) == (x R1 y)  (x R2 y)
	Thm* T:Type, R1,R2:(TTProp). (R1  R2)  TTProp

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