unified access to retrieve SymbolType of grammar

alib2data/src/grammar/Grammar.h

@@ -11,6 +11,8 @@
 #include <base/WrapperBase.hpp>
 #include "GrammarBase.h"
+#include <type_traits>
+
 namespace grammar {
 /**
@@ -27,6 +29,9 @@ public:
 	}
 };
+template < class T >
+using SymbolTypeOfGrammar = typename std::decay < decltype (std::declval<T>().getNonterminalAlphabet()) >::type::value_type;
+
 } /* namespace grammar */
 #endif /* GRAMMAR_H_ */