diff --git a/src/kernel/converter.cpp b/src/kernel/converter.cpp
index 105cca3e1..fbb234f18 100644
--- a/src/kernel/converter.cpp
+++ b/src/kernel/converter.cpp
@@ -13,6 +13,35 @@ Author: Leonardo de Moura
 #include "kernel/type_checker.h"
 
 namespace lean {
+/**
+   \brief Predicate for deciding whether \c d is an opaque definition or not.
+
+   Here is the basic idea:
+
+   1) Each definition has an opaque flag. This flag cannot be modified after a definition is added to the environment.
+   The opaque flag affects the convertability check. The idea is to minimize the number of delta-reduction steps.
+   We also believe it increases the modularity of Lean developments by minimizing the dependency on how things are defined.
+   We should view non-opaque definitions as "inline definitions" used in programming languages such as C++.
+
+   2) Whenever type checking an expression, the user can provide an additional set of definition names (m_extra_opaque) that
+   should be considered opaque. Note that, if \c t type checks when using an extra_opaque set S, then t also type checks
+   (modulo resource constraints) with the empty set. Again, the purpose of extra_opaque is to mimimize the number
+   of delta-reduction steps.
+
+   3) To be able to prove theorems about an opaque definition, we treat an opaque definition D in a module M as
+   transparent when we are type checking another definition/theorem D' also in M. This rule only applies if
+   D is not a theorem, nor D is in the set m_extra_opaque. To implement this feature, this class has a field
+   m_module_idx that is not none when this rule should be applied.
+*/
+bool is_opaque(declaration const & d, name_set const & extra_opaque, optional<module_idx> const & mod_idx) {
+    lean_assert(d.is_definition());
+    if (d.is_theorem()) return true;                               // theorems are always opaque
+    if (extra_opaque.contains(d.get_name())) return true;          // extra_opaque set overrides opaque flag
+    if (!d.is_opaque()) return false;                              // d is a transparent definition
+    if (mod_idx && d.get_module_idx() == *mod_idx) return false;   // the opaque definitions in mod_idx are considered transparent
+    return true;                                                   // d is opaque
+}
+
 static no_delayed_justification g_no_delayed_jst;
 bool converter::is_def_eq(expr const & t, expr const & s, type_checker & c) {
     return is_def_eq(t, s, c, g_no_delayed_jst);
@@ -138,33 +167,8 @@ struct default_converter : public converter {
         return r;
     }
 
-    /**
-       \brief Predicate for deciding whether \c d is an opaque definition or not.
-
-       Here is the basic idea:
-
-       1) Each definition has an opaque flag. This flag cannot be modified after a definition is added to the environment.
-       The opaque flag affects the convertability check. The idea is to minimize the number of delta-reduction steps.
-       We also believe it increases the modularity of Lean developments by minimizing the dependency on how things are defined.
-       We should view non-opaque definitions as "inline definitions" used in programming languages such as C++.
-
-       2) Whenever type checking an expression, the user can provide an additional set of definition names (m_extra_opaque) that
-       should be considered opaque. Note that, if \c t type checks when using an extra_opaque set S, then t also type checks
-       (modulo resource constraints) with the empty set. Again, the purpose of extra_opaque is to mimimize the number
-       of delta-reduction steps.
-
-       3) To be able to prove theorems about an opaque definition, we treat an opaque definition D in a module M as
-       transparent when we are type checking another definition/theorem D' also in M. This rule only applies if
-       D is not a theorem, nor D is in the set m_extra_opaque. To implement this feature, this class has a field
-       m_module_idx that is not none when this rule should be applied.
-    */
     bool is_opaque(declaration const & d) const {
-        lean_assert(d.is_definition());
-        if (d.is_theorem()) return true;                                       // theorems are always opaque
-        if (m_extra_opaque.contains(d.get_name())) return true;                // extra_opaque set overrides opaque flag
-        if (!d.is_opaque()) return false;                                      // d is a transparent definition
-        if (m_module_idx && d.get_module_idx() == *m_module_idx) return false; // the opaque definitions in module_idx are considered transparent
-        return true;                                                           // d is opaque
+        return ::lean::is_opaque(d, m_extra_opaque, m_module_idx);
     }
 
     /** \brief Expand \c e if it is non-opaque constant with weight >= w */
diff --git a/src/kernel/converter.h b/src/kernel/converter.h
index cd2dabe27..466e90f53 100644
--- a/src/kernel/converter.h
+++ b/src/kernel/converter.h
@@ -30,4 +30,6 @@ std::unique_ptr<converter> mk_default_converter(environment const & env,
                                                 name_set const & extra_opaque = name_set());
 std::unique_ptr<converter> mk_default_converter(environment const & env, bool unfold_opaque_main,
                                                 bool memoize = true, name_set const & extra_opaque = name_set());
+
+bool is_opaque(declaration const & d, name_set const & extra_opaque, optional<module_idx> const & mod_idx);
 }