fix small bug in FGrammarLog

src/FGrammarLog.gf

@@ -25,7 +25,7 @@ concrete FGrammarLog of FGrammar = MCatsLog ** {
         };
         fcid_fbinrel_fexpr_to_identifier a r1 b = {
             formula = r1 ++ "(" ++ a ++ "," ++ b ++ ")";
-            core = b;
+            core = a;
             hasFormula = true;
         };
         fexpr_fbinrel_fcid_fbinrel_fexpr_to_identifier a r1 b r2 c = {