i don't complain about different units if one of them is ""

pub/src/func.cpp

@@ -352,7 +352,9 @@ CCoord CFunc::coord(class CCoord *co1, class CCoord *co2) const
 
     if (s=="+" || s=="-") {
         co.name = co1->name + s + co2->name;
-        if (co2->unit != co1->unit) {
+        if ( co1->unit=="" )
+            co.unit = "";
+        else if ( co2->unit!="" && co2->unit != co1->unit) {
             cout << "? different units " << co1->unit << " and " <<
                 co2->unit << " in additive operation\n";
             co.unit = "";