[CM] strange assertion failure and negative pitches with MOVING-SPECTRUM in snd 19.6

[bil at ccrma.Stanford.EDU]

You get the assertion failure because (format () ...)
sends its output to the current output; I think you
want (format #f ...) which just returns a string.
There is an assert macro in s7.html.

The "freqs" are probably negative because they are
phase increments ("instantaneous frequency") (in radians).
As in the phase-vocoder, it's using a sliding fft
and returning data for reconstruction as the
windows go by.  pins in clm-ins.scm shows one way
to use this kind of data.  The test-sv function
in generators.scm shows the parallel.  sine-bank
below that function shows a direct use.