diff --git a/src/ast/datatype_decl_plugin.cpp b/src/ast/datatype_decl_plugin.cpp
index 44f824959..f685c70d5 100644
--- a/src/ast/datatype_decl_plugin.cpp
+++ b/src/ast/datatype_decl_plugin.cpp
@@ -821,6 +821,10 @@ namespace datatype {
         return d;
     }
 
+    app* util::mk_is(func_decl * c, expr *f) {
+        return m.mk_app(get_constructor_is(c), 1, &f);
+    }
+
     func_decl * util::get_recognizer_constructor(func_decl * recognizer) const {
         SASSERT(is_recognizer(recognizer));
         return to_func_decl(recognizer->get_parameter(0).get_ast());
diff --git a/src/ast/datatype_decl_plugin.h b/src/ast/datatype_decl_plugin.h
index e644ccbda..f32e9990d 100644
--- a/src/ast/datatype_decl_plugin.h
+++ b/src/ast/datatype_decl_plugin.h
@@ -362,6 +362,7 @@ namespace datatype {
         bool is_recognizer(app * f) const { return is_recognizer0(f) || is_is(f); }
         bool is_accessor(app * f) const { return is_app_of(f, m_family_id, OP_DT_ACCESSOR); }
         bool is_update_field(app * f) const { return is_app_of(f, m_family_id, OP_DT_UPDATE_FIELD); }
+        app* mk_is(func_decl * c, expr *f);
         ptr_vector<func_decl> const * get_datatype_constructors(sort * ty);
         unsigned get_datatype_num_constructors(sort * ty);
         unsigned get_datatype_num_parameter_sorts(sort * ty);
diff --git a/src/ast/rewriter/datatype_rewriter.cpp b/src/ast/rewriter/datatype_rewriter.cpp
index f0a95929b..194668b9c 100644
--- a/src/ast/rewriter/datatype_rewriter.cpp
+++ b/src/ast/rewriter/datatype_rewriter.cpp
@@ -23,6 +23,9 @@ br_status datatype_rewriter::mk_app_core(func_decl * f, unsigned num_args, expr
     switch(f->get_decl_kind()) {
     case OP_DT_CONSTRUCTOR: return BR_FAILED;
     case OP_DT_RECOGNISER:
+        SASSERT(num_args == 1);
+        result = m_util.mk_is(m_util.get_recognizer_constructor(f), args[0]);
+        return BR_REWRITE1;
     case OP_DT_IS:
         //
         // simplify is_cons(cons(x,y)) -> true
diff --git a/src/interp/iz3interp.cpp b/src/interp/iz3interp.cpp
index 3d1d84f3c..41c968bd8 100644
--- a/src/interp/iz3interp.cpp
+++ b/src/interp/iz3interp.cpp
@@ -255,12 +255,12 @@ public:
         catch (const char *msg) {
             throw interpolation_failure(msg);
         }
-        catch (const iz3translation::unsupported &) {
-            TRACE("iz3", tout << "unsupported\n";);
+        catch (const iz3translation::unsupported & ex) {
+            TRACE("iz3", tout << "unsupported " << "\n";);
             throw interpolation_error();
         }
-        catch (const iz3proof::proof_error &) {
-            TRACE("iz3", tout << "proof error\n";);
+        catch (const iz3proof::proof_error & ex) {
+            TRACE("iz3", tout << "proof error " << "\n";);
             throw interpolation_error();
         }
         profiling::timer_stop("Proof translation");
@@ -306,8 +306,8 @@ public:
             catch (const char *msg) {
                 throw interpolation_failure(msg);
             }
-            catch (const iz3translation::unsupported &) {
-                TRACE("iz3", tout << "unsupported\n";);
+            catch (const iz3translation::unsupported & ex) {
+                TRACE("iz3", tout << "unsupported " << "\n";);
                 throw interpolation_error();
             }
             catch (const iz3proof::proof_error &) {
diff --git a/src/interp/iz3translate.cpp b/src/interp/iz3translate.cpp
index 9680ec95e..44bac0643 100644
--- a/src/interp/iz3translate.cpp
+++ b/src/interp/iz3translate.cpp
@@ -2029,8 +2029,8 @@ public:
             case PR_IFF_FALSE: { // turns ~p into p <-> false, noop for us
                 if(is_local(con))
                     res = args[0];
-                else
-                    throw_unsupported(con);
+                else 
+                    throw_unsupported(proof);
                 break;
             }
             case PR_COMMUTATIVITY: {