diff --git a/contrib/HoTTBook.v b/contrib/HoTTBook.v
index 9c02d6a8534..4599dae0d2b 100644
--- a/contrib/HoTTBook.v
+++ b/contrib/HoTTBook.v
@@ -602,7 +602,7 @@ Definition Book_4_3_1  := @HoTT.Equiv.BiInv.BiInv.
 (* ================================================== thm:isprop-biinv *)
 (** Theorem 4.3.2 *)
 
-Definition Book_4_3_2  := @HoTT.Equiv.BiInv.isprop_biinv.
+Definition Book_4_3_2  := @HoTT.Equiv.BiInv.ishprop_biinv.
 
 (* ================================================== thm:equiv-biinv-isequiv *)
 (** Corollary 4.3.3 *)
diff --git a/contrib/HoTTBookExercises.v b/contrib/HoTTBookExercises.v
index 7072589c487..1ecc72dc2bb 100644
--- a/contrib/HoTTBookExercises.v
+++ b/contrib/HoTTBookExercises.v
@@ -1016,12 +1016,10 @@ Section Book_4_5.
     Proof.
       apply isequiv_biinv.
       split.
-      - exists ((h o g)^-1 o h);
-        repeat intro; simpl;
-        try apply (@eissect _ _ (h o g)).
-      - exists (f o (g o f)^-1);
-        repeat intro; simpl;
-        try apply (@eisretr _ _ (g o f)).
+      - exists ((h o g)^-1 o h).
+        exact (eissect (h o g)).
+      - exists (f o (g o f)^-1).
+        exact (eisretr (g o f)).
     Defined.
 
     Local Instance Book_4_5_f : IsEquiv f.
diff --git a/theories/Equiv/BiInv.v b/theories/Equiv/BiInv.v
index 1cce87bc936..3861cbfe5c9 100644
--- a/theories/Equiv/BiInv.v
+++ b/theories/Equiv/BiInv.v
@@ -19,11 +19,26 @@ Definition isequiv_biinv `(f : A -> B)
 Proof.
   intros [[g s] [h r]].
   exact (isequiv_adjointify f g
-    (fun x => ap f (ap g (r x)^ @ s (h x))  @ r x)
+    (fun x => ap f (ap g (r x)^ @ s (h x)) @ r x)
     s).
 Defined.
 
-Global Instance isprop_biinv `{Funext} `(f : A -> B) : IsHProp (BiInv f) | 0.
+Definition biinv_isequiv `(f : A -> B)
+  : IsEquiv f -> BiInv f.
+Proof.
+  intros [g s r adj].
+  exact ((g; r), (g; s)).
+Defined.
+
+Definition iff_biinv_isequiv `(f : A -> B)
+  : BiInv f <-> IsEquiv f.
+Proof.
+  split.
+  - apply isequiv_biinv.
+  - apply biinv_isequiv.
+Defined.
+
+Global Instance ishprop_biinv `{Funext} `(f : A -> B) : IsHProp (BiInv f) | 0.
 Proof.
   apply hprop_inhabited_contr.
   intros bif; pose (fe := isequiv_biinv f bif).
@@ -36,9 +51,5 @@ Defined.
 Definition equiv_biinv_isequiv `{Funext} `(f : A -> B)
   : BiInv f <~> IsEquiv f.
 Proof.
-  apply equiv_iff_hprop.
-  - by apply isequiv_biinv.
-  - intros ?.  split.
-    + by exists (f^-1); apply eissect.
-    + by exists (f^-1); apply eisretr.
+  apply equiv_iff_hprop_uncurried, iff_biinv_isequiv.
 Defined.
diff --git a/theories/Sets/GCHtoAC.v b/theories/Sets/GCHtoAC.v
index a99631af6a6..63fec8f30be 100644
--- a/theories/Sets/GCHtoAC.v
+++ b/theories/Sets/GCHtoAC.v
@@ -8,17 +8,15 @@ From HoTT.Sets Require Import Ordinals Hartogs Powers GCH AC.
 
 Open Scope type.
 
-(* The proof of Sierpinski's results that GCH implies AC given in this file consists of 2 ingredients:
+(* The proof of Sierpinski's results that GCH implies AC given in this file consists of two ingredients:
    1. Adding powers of infinite sets does not increase the cardinality (path_infinite_power).
    2. A variant of Cantor's theorem saying that P(X) <= (X + Y) implies P(X) <= Y for large X (Cantor_injects_injects).
-   Those are used to obtain that cardinality-controlled functions are well-behaved in the presence of GCH (Sierpinski),
-   from which we obtain by instantiation with the Hartogs number that every set embeds into an ordinal,
-   which is enough to conclude GCH -> AC (GCH_AC) since the well-ordering theorem implies AC (WO_AC). *)
+   Those are used to obtain that cardinality-controlled functions are well-behaved in the presence of GCH (Sierpinski), from which we obtain by instantiation with the Hartogs number that every set embeds into an ordinal, which is enough to conclude GCH -> AC (GCH_AC) since the well-ordering theorem implies AC (WO_AC). *)
 
 
 (** * Constructive equivalences *)
 
-(* For the 1. ingredient, we establish a bunch of paths and conclude the wished result by equational reasoning. *)
+(* For the first ingredient, we establish a bunch of paths and conclude the desired result by equational reasoning. *)
 
 Section Preparation.
 
@@ -63,20 +61,10 @@ Proof.
 Qed.
 
 
-
 (** * Equivalences relying on LEM **)
 
 Context {EM : ExcludedMiddle}.
 
-Lemma PE (P P' : HProp) :
-  P <-> P' -> P = P'.
-Proof.
-  intros [f g]. apply path_trunctype.
-  exists f. apply isequiv_biinv. split.
-  - exists g. intros x. apply P.
-  - exists g. intros x. apply P'.
-Qed.
-
 Lemma path_bool_prop :
   HProp = Bool.
 Proof.
@@ -86,7 +74,7 @@ Proof.
   - intros []; destruct LEM as [H|H]; auto.
     + destruct (H (tr tt)).
     + apply (@merely_destruct Empty); easy.
-  - intros P. destruct LEM as [H|H]; apply PE.
+  - intros P. destruct LEM as [H|H]; apply equiv_path_iff_hprop.
     + split; auto. intros _. apply tr. exact tt.
     + split; try easy. intros HE. apply (@merely_destruct Empty); easy.
 Qed.
@@ -125,7 +113,6 @@ Proof.
 Qed.
 
 
-
 (** * Equivalences on infinite sets *)
 
 Lemma path_infinite_unit (X : HSet) :
@@ -144,11 +131,9 @@ Proof.
 Qed.
 
 
-
 (** * Variants of Cantors's theorem *)
 
-(* For the 2. ingredient, we give a preliminary version (Cantor_path_inject) to see the idea,
-   as well as a stronger refinement (Cantor_injects_injects) which is then a mere reformulation. *)
+(* For the second ingredient, we give a preliminary version (Cantor_path_inject) to see the idea, as well as a stronger refinement (Cantor_injects_injects) which is then a mere reformulation. *)
 
 Context {PR : PropResizing}.
 
@@ -213,7 +198,7 @@ Lemma InjectsInto_power_morph X Y :
 Proof.
   intros HF. eapply merely_destruct; try apply HF. intros [f Hf].
   apply tr. exists (fun p => fun y => hexists (fun x => p x /\ y = f x)).
-  intros p q H. apply path_forall. intros x. apply PE. split; intros Hx.
+  intros p q H. apply path_forall. intros x. apply equiv_path_iff_hprop. split; intros Hx.
   - assert (Hp : (fun y : Y => hexists (fun x : X => p x * (y = f x))) (f x)). { apply tr. exists x. split; trivial. }
     pattern (f x) in Hp. rewrite H in Hp. eapply merely_destruct; try apply Hp. now intros [x'[Hq <- % Hf]].
   - assert (Hq : (fun y : Y => hexists (fun x : X => q x * (y = f x))) (f x)). { apply tr. exists x. split; trivial. }
@@ -242,7 +227,6 @@ Qed.
 End Preparation.
 
 
-
 (** * Sierpinski's Theorem *)
 
 Section Sierpinski.
@@ -261,9 +245,7 @@ Hypothesis HN_ninject : forall X, ~ InjectsInto (HN X) X.
 Variable HN_bound : nat.
 Hypothesis HN_inject : forall X, InjectsInto (HN X) (power_iterated X HN_bound).
 
-(* This section then concludes the intermediate result that abstractly,
-   any function HN behaving like the Hartogs number is tamed in the presence of GCH.
-   Morally we show that X <= HN(X) for all X, we just ensure that X is large enough by considering P(N + X). *)
+(* This section then concludes the intermediate result that abstractly, any function HN behaving like the Hartogs number is tamed in the presence of GCH.  Morally we show that X <= HN(X) for all X, we just ensure that X is large enough by considering P(N + X). *)
 
 Lemma InjectsInto_sum X Y X' Y' :
   InjectsInto X X' -> InjectsInto Y Y' -> InjectsInto (X + Y) (X' + Y').
@@ -279,8 +261,7 @@ Proof.
   - apply ap. apply Hg. apply path_sum_inr with X'. apply H.
 Qed.
 
-(* The main proof is by induction on the cardinality bound for HN.
-   As the Hartogs number is bounded by P^3(X), we'd actually just need finitely many instances of GCH. *)
+(* The main proof is by induction on the cardinality bound for HN.  As the Hartogs number is bounded by P^3(X), we'd actually just need finitely many instances of GCH. *)
 
 Lemma Sierpinski_step (X : HSet) n :
   GCH -> infinite X -> powfix X -> InjectsInto (HN X) (power_iterated X n) -> InjectsInto X (HN X).
diff --git a/theories/TruncType.v b/theories/TruncType.v
index 3a094de96b0..179df606a4b 100644
--- a/theories/TruncType.v
+++ b/theories/TruncType.v
@@ -163,4 +163,10 @@ Section TruncType.
     exact (Build_Equiv _ _ path_iff_ishprop_uncurried _).
   Defined.
 
+  Lemma equiv_path_iff_hprop {A B : HProp}
+    : (A <-> B) <~> (A = B).
+  Proof.
+    refine (equiv_path_trunctype' _ _ oE equiv_path_iff_ishprop).
+  Defined.
+
 End TruncType.