Actually, this is what the XML collection already does.  You can
write, say,

  (p ()
    "There is already a "
    "well-developed theory of "
    (em () "expressive")
    " language constructs")

and the renderer decides what to do with the multiple strings.
(xml->string concatenates them, a direct renderer of the sort Oleg
mentions can write them straight out to a port).

'shriram