---------------------------------------
x x /ext_link_grp1/x_dset
x x /ext_link_grp2/x_dset
dataset: </ext_link_dset1> and </ext_link_dset1>
dataset: </ext_link_dset2> and </ext_link_dset2>
group : </ext_link_grp1> and </ext_link_grp1>
dataset: </ext_link_grp1/x_dset> and </ext_link_grp1/x_dset>
group : </ext_link_grp2> and </ext_link_grp2>
dataset: </ext_link_grp2/x_dset> and </ext_link_grp2/x_dset>
dangling link: </ext_link_noexist1> and </ext_link_noexist1>
dangling link: </ext_link_noexist2> and </ext_link_noexist2>