I once found this service and cannot find it again.
Could it be that you are looking for: http://www.openoffice.org/api/docs/common/ref/com/sun/star/ui/dialogs/XFilePicker.html#getFiles?
Thanks! However, my question was not about the file picker. It was a service to which I could pass an accessible folder in URL notation and which returned the contained elements. The result was called a sequence but actually was a 1D-array.
I got it again. It was the service com.sun.star.ucb.SimpleFileAccess.
My question specifically addressed one of many member functions (of the interface):
sequence< string > getFolderContents ([in] string FolderURL, [in] boolean bIncludeFolders)
In LibreOffice Basic the service is accessible via an object created by createUnoService("com.sun.star.ucb.SimpleFileAccess")