// User should ask gdcm to write an image in Explicit VR mode

// User should ask gdcm to write an image in Explicit VR mode
    // only when he is sure the VR of *all* the DataElements is known.
    // i.e : when there are *only* Public Groups,
    // or *all* the Shadow Groups are fully described in
    // the relevant Shadow Dictionnary.
    // Let's just *dream* about it; *never* trust a user !
    // If he doesn't, it write process will produce an 'illegal' Dicom image.
    // (it doesn't bother most of Dicom readers, but ...
    // Therae are enought illegal Dicom image in the world.
    // Don't add illegal gdcm-written Dicom images !

In order to avoid furter troubles,
We turn to Implicit VR if at least the VR of one element is unknown.