+/// \brief TagKey is made to hold an "universal" (as in URL, Universal
+/// Ressource Locator) key to a DocEntry i.e. a dicom tag.
+/// A dicom tag always has a group and an elem, but a set of tags
+/// embeded in various (optionally nested) sequences and sharing
+/// the same group and elem all share the same (group, elem)
+/// "identifier". Hence the (group, elem) cannot be used as an
+/// identifier (in gdcm we shall refer to a "TagKey") of a tag.
+/// In order to construct a proper tag identifier (i.e. a key) we
+/// consider the following definition of a TagKey:
+/// - let Group, Element be the string representation of the
+/// group and elem dicom tag members,
+/// - let ItemNumber be the string representation of the integer
+/// index of the considered item number of a sequence,
+/// Let the key of a tag embeded in a sequence, noted SeqTag, be
+/// the form:
+/// /ItemNumber#Group|Element
+/// where "/", "#" and "|" are characters acting as separators.
+/// Then the general form of a TagKey is given by:
+/// Group|Element[SeqTag]
+/// where [SeqTag] means NO or many instances of SeqTag.
+/// Hence the TagKey of a tag not "leaving" in a sequence is the
+/// string e.g.
+/// 0028|1201
+/// but the TagKey of a tag "embeded" is the first item of
+/// a sequence, itself nested in the third item of a sequence is the
+/// string e.g.
+/// 0004|1220/2#0008|0082/0#0008|0090