The HDF_VD_ISVD function returns True (1) if the object associated with Id is a VData in a Hierarchical Data Format file.
Result = HDF_VD_ISVD( VGroup, Id )
The VGroup handle returned by a previous call to HDF_VG_ATTACH.
The VGroup or VData reference number obtained by a previous call to HDF_VG_GETNEXT or HDF_VD_GETNEXT.
Vid = HDF_VD_GETNEXT(Vgrp, -1)
PRINT, HDF_VD_ISVD(VGrp, Vid)