         To get it done we must get only unique built-in expressions that are not 
         in loaded expressions.
         */
-        let uniqueBuiltInExpressions = builtInExpressions.filter({
-            let builtIn = $0
-            return loadedExpressions.filter({$0.identifier == builtIn.identifier}).count == 0
-        })
-        
-        return loadedExpressions + uniqueBuiltInExpressions
+        return loadedExpressions + (builtInExpressions <! loadedExpressions)
     }
     
     /**

        }
    }
}

        and this is good as base expression.
        2. He forgot to define such expression for prefered language.
        */
        let uniqueBaseExpressions = baseLanguageExpressions <! preferedLanguageExpressions
        
        // Expressions from json files.
        var loadedExpressions = uniqueBaseExpressions + preferedLanguageExpressions
        
        // Load prefered language nad base built-in expressions. Get unique.
        let prefBuiltInExpressions = loadBuiltInExpressions(preferedLanguage)
        let baseBuiltInExpressions = SharedBaseExpression.allExpressions()
        let uniqueBaseBuiltInExpressions = baseBuiltInExpressions <! prefBuiltInExpressions
        
        // Unique built-in expressions made of base + prefered language.
        let builtInExpressions = uniqueBaseBuiltInExpressions + prefBuiltInExpressions
