diff --git a/include/autoq/aut_description.hh b/include/autoq/aut_description.hh
index 369312dc58..97c68730e6 100644
--- a/include/autoq/aut_description.hh
+++ b/include/autoq/aut_description.hh
@@ -134,7 +134,7 @@ public:   // methods
 
 	/******************************************************/
     /* inclusion.cc: checks language inclusion of two TAs */
-    bool operator<=(const Automata &o) const requires concrete_like<TT>;
+    bool operator<=(const Automata &o) const; // requires concrete_like<TT>;
     bool operator>=(const Automata &o) const requires concrete_like<TT> { return o <= *this; }
 	bool operator==(const Automata &o) const requires concrete_like<TT> { return (*this <= o) && (o <= *this); }
     bool operator!=(const Automata &o) const requires concrete_like<TT> { return !(*this == o); }
diff --git a/src/inclusion.cc b/src/inclusion.cc
index d9de32c303..1a8ead299c 100644
--- a/src/inclusion.cc
+++ b/src/inclusion.cc
@@ -382,6 +382,9 @@ bool AUTOQ::Automata<AUTOQ::Symbol::Concrete>::operator<=(const Automata<AUTOQ::
     return result;
 }
 
+template <typename Symbol>
+bool AUTOQ::Automata<Symbol>::operator<=(const Automata<Symbol> &autB) const {}
+
 bool AUTOQ::check_validity(Constraint C, const PredicateAutomata::Symbol &ps, const SymbolicAutomata::Symbol &te) {
     std::string str(ps);
     /* Replace all real(.) in C.content with .R and