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Copied From Coq.Logic.JMeq.

No longer involving JMeq_eq, which is potentially inconsistenty with homotopy
type theory.

TODO: let reflexivity, symmetry and transitivity work again.

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Definition JMeq {A:Type} (x:A) {B:Type} (y: B): Prop :=
  {H: @eq Type A B | @eq_rect Type A (fun T: Type => T) x B H = y}.

Lemma JMeq_eq : forall (A:Type) (x y:A), JMeq x y -> x = y.