Building up a hierarchy from a flat structure

Dear colleagues,

I have an XML file with a set of entries, each containing a sequence of 
elements. I have to introduce container elements that group some of these 
elements together. The A element is intended as a hook where I want to start 
grouping all the following-siblings so that all following-siblings after a 
specific A are grouped together as children of A until the next A or the end of 
the ENTRY is reached. One problem is that I want to keep the sequence of 
elements as in the original and do not know what elements in which frequency 
and in what sequence occur in my data, the second problem is that I don't have 
the A hook at the end of the ENTRY. How would such a procedural logics (start 
at a certain element to move each following element in turn into a container 
element until a certain element is reached) be implemented in XSLT?

The sample files have been simplified. In the original the elements are not 
empty, but contain substructure.

See in.xml (the input) and out.xml(the intended output).

IN.XML: Input file

<?xml version="1.0" encoding="UTF-8"?>
<ROOT>
        <ENTRY>
                <X/>
                <A/>
                <SG/>
                <TG/>
                <TG/>
                <U/>
                <U/>
                <U/>
                <TG/>
                <A/>
                <SG/>
                <TG/>
        </ENTRY>
        <ENTRY>
                <B/>
                <A/>
                <SG/>
                <TG/>
                <SG/>
                <TG/>
                <TG/>
                <TG/>
                <SG/>
                <TG/>
                <A/>
                <SG/>
                <TG/>
        </ENTRY>
        <ENTRY>
                <C/>
                <A/>
                <SG/>
                <U/>
                <SG/>
                <TG/>
                <A/>
                <SG/>
                <U/>
                <U/>
                <U/>
                <U/>
                <U/>
        </ENTRY>
        <ENTRY>
                <Y/>
                <A/>
                <SG/>
                <U/>
                <TG/>
                <U/>
                <TG/>
                <A/>
                <SG/>
                <TG/>
                <TG/>
                <TG/>
        </ENTRY>
        <ENTRY>
                <C/>
                <Z/>
        </ENTRY>
</ROOT>

OUT.XML: Desired Output

<?xml version="1.0" encoding="UTF-8"?>
<ROOT>
        <ENTRY>
                <X/>
                <A>
                        <SG/>
                        <TG/>
                        <TG/>
                        <U/>
                        <U/>
                        <U/>
                        <TG/>
                </A>
                <A>
                        <SG/>
                        <TG/>
                </A>
        </ENTRY>
        <ENTRY>
                <B/>
                <A>
                        <SG/>
                        <TG/>
                        <SG/>
                        <TG/>
                        <TG/>
                        <TG/>
                        <SG/>
                        <TG/>
                </A>
                <A>
                        <SG/>
                        <TG/>
                </A>
        </ENTRY>
        <ENTRY>
                <C/>
                <A>
                        <SG/>
                        <U/>
                        <SG/>
                        <TG/>
                </A>
                <A>
                        <SG/>
                        <U/>
                        <U/>
                        <U/>
                        <U/>
                        <U/>
                </A>
        </ENTRY>
        <ENTRY>
                <Y/>
                <A>
                        <SG/>
                        <U/>
                        <TG/>
                        <U/>
                        <TG/>
                </A>
                <A>
                        <SG/>
                        <TG/>
                        <TG/>
                        <TG/>
                </A>
        </ENTRY>
        <ENTRY>
                <C/>
                <Z/>
        </ENTRY>
</ROOT>

Many thanks in advance for your patience and your kind assistance!

Best regards
Wolfgang


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