Generating tones with alternating frequencies

Hello,

first I'd like to use the occasion and thank the programmers, who is
developing Sox, for their amazing work!

And here is my question.

Sox allows one to generate a tone whose frequency changes from freq1 to freq2:

play -n synth 4 sine 200-400

If one needs the tone to oscillate between these frequencies, this can be 
achieved e.g. with the following:

play -n synth 4 sine 200:400 sine 400:200 delay 0 4 remix 1,2 1,2 repeat 999

The tone will oscillate between 200 and 400 Hz with the 4 second period.

With even more trickery one can achieve that several tones simultaneously
oscillate between different frequencies with *different* periods. That is
one tone alternates between frequencies more faster than another.

What I cannot figure out how to achieve (and this is my question) is the
following:

is it possible with Sox to generate a stream where several tones oscillate
between different frequencies with different periods **and** these periods
are themselves oscillate (that is for each tone the speed of alternating 
between frequencies constantly changes back and forth between some given
boundaries)?


Albert.