[O] parameterizing keyword values during a #+call

[Greg Minshall]

hi.  i would have an org file that uses lots of #+calls to various
asymptote routines (babelled into the file) to produce graphics.  i
would like to export this file to both html and to latex (specifically
beamer).  for html, i would like, e.g., SVG files, and for latex, PDF
files.  the file name is normally passed to the asymptote routine via a
:file parameter, to wit
----
#+call: disc[:file images/disc.svg]() :results file
----
and the file extension (".svg" in the above) determines the format of
the asymptote output.

is there an obvious way to allow the file extension to take on different
values depending on which sort of export is driving its execution?  i
had hoped that replacing ".svg" with something like ".{{{ext()}}}",
where "ext" was a macro which selected amongst the alternatives(*),
would do the trick.  but that doesn't appear to be the case.

thanks in advance (and with full appreciation of what a pain macros
are to implement fully, halfly, anyly), Greg Minshall

(*)
#+MACRO: ext @@latex:pdf@@@@beamer:pdf@@@@html:svg@@