Put fmap sorting lemmas in a section.

theories/sorting.v

@@ -96,6 +96,7 @@ Section sorted.
     end); clear go; abstract first [by constructor | by inversion 1].
   Defined.
 
+  Section fmap.
     Context {B} (f : A → B).
     Lemma HdRel_fmap (R1 : relation A) (R2 : relation B) x l :
       (∀ y, R1 x y → R2 (f x) (f y)) → HdRel R1 x l → HdRel R2 (f x) (f <$> l).
@@ -110,6 +111,7 @@ Section sorted.
       induction 2; csimpl; constructor;
         rewrite ?Forall_fmap; eauto using Forall_impl.
     Qed.
+  End fmap.
 End sorted.
 
 (** ** Correctness of merge sort *)