allow retrieval of regexp's symbol type

alib2data/src/regexp/RegExp.h

@@ -35,6 +35,9 @@ class RegExp : public base::WrapperBase < RegExpBase > {
 };
+template < class T >
+using SymbolTypeOfRegexp = typename std::decay < decltype (std::declval<T>().getAlphabet()) >::type::value_type;
+
 regexp::RegExp regexpFrom ( const std::string & string );
 regexp::RegExp regexpFrom ( const char * string );