fix an invariant violation in the previous commit:

the [Duse] declaration are preserved on Context.use_export,
so that ctxt_known can be always restored from ctxt_decls.

src/core/theory.ml

@@ -443,7 +443,7 @@ module Context = struct
     let d = create_use th in
     try
       let kn = add_known ctxt.ctxt_known th.th_name d in
-      let ctxt = Hctxt.hashcons { ctxt with ctxt_known = kn } in
+      let ctxt = push_kn_decl ctxt kn d in
       let add_decl ctxt d = match d.d_node with
         | Dtype _ | Dlogic _ | Dprop _ -> add_decl ctxt d
         | Duse th -> use_export ctxt th

src/test.why

 
 (* test file *)
 
-theory Test
-  inductive p(int) =
-  | A : 0=0
+theory ThA
+    type 'a t = None | Some ('a)
+    type s
+    logic c : s
 end
 
-theory TestPrelude 
-  use prelude.Int
-  use prelude.List
+theory ThB
+    use export prelude.List
+    clone ThA with type s = int
+end
+
+theory ThC
 end