matching elements of a list

do I have reached the limits of xpath?

if several linked lists are contained in one document how to match all
nodes belonging to a specific start node?

The following example may illustrate my question:

Input:
<root>
    <st ix="a"/>
    <st ix="b"/>
    <el ix="c"/>
    <el ix="d"/>
    <el ix="e"/>
    <el ix="f"/>
    <el ix="g"/>

    <ln s="a" d="g"/>
    <ln s="g" d="f"/>

    <ln s="b" d="e"/>
    <ln s="e" d="c"/>
    <ln s="c" d="d"/>
</root>

disired output:

a: g f
b: e c d

How to accomplish that by XSLT (2.0)?
Even your answer "I know for sure that there is not elegant way"
or "You got to extend the processor by some Java function"
would help.