[xsl] Join elements by name, with spaces in between

[Yves Forkl <Y.Forkl@xxxxxx>]

Hi,

in XSLT 2.0 I want to join adjacent sibling elements of the same name, 
and spaces in between should be included on this occasion. The elements 
may appear in any order. For instance, if a, b and c instances may need 
joining, whereas x instances do never repeat and thus are never joined 
(this is meant to be one single line):

<e><x>x</x> <a>a1</a> <a>a2</a> <b>b</b> <c>c1</c> <c>c2</c> <a>a3</a></e>

should become

<e><x>x</x> <a>a1 a2</a> <b>b</b> <c>c1 c2</c> <a>a3</a></e>

My idea is to use for-each-group, but I can't figure out how to process 
the siblings and the possible space in between only once and in the 
right order.

Can you help me?

  Yves