fix comparison

bfps/cpp/particles/abstract_particles_system.hpp

@@ -80,7 +80,7 @@ public:
     void completeLoopWithExtraField(
             const real_number dt,
             const field<rnumber, be, fc>& in_field) {
-        static_assert(fc == THREE || THREExTHREE, "only THREE or THREExTHREE is supported for now");
+        static_assert((fc == THREE) || (fc == THREExTHREE), "only THREE or THREExTHREE is supported for now");
         if (fc == THREE)
         {
             std::unique_ptr<real_number[]> extra_rhs(new real_number[getLocalNbParticles()*3]());