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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_sym : forall {A: Type} {B:Type} {x:A} {y:B}, JMeq x y -> JMeq y x.