VT_*** [constants]
VT_NULL, VT_BOOL, VT_INT, VT_REAL, VT_STRING, VT_FUNC, VT_CFUNC, VT_OBJECT, VT_PTR, VT_THREAD
these constants are defined to the values of the C macros (with the prefix "SGS_VT_" in C) and can be compared with the values returned in typeid()