shepenv - special generator for Shepard/Risset tone spectral envelopes

	gen T shepenv F   cycles base

shepenv generates a function suitable for use with the illus unit
generator in the production of Shepard or Risset illusions.  The
function consists of a "raised inverted cosine" waveform that begins
and ends with the value 0.0, and attains a single peak of 1.0 in
between (i.e., on a linear scale, f(x) = .5 * -cos(x) + .5, for 0 <= x
<= 2pi).  Shepenv scales this function along a logarithmic abscissa, so
that equal pitch intervals occupy equal intervals along the horizontal
axis.  Shepenv accepts 2 arguments: the number of logarithmic cycles,
and the logarithm base.

For example, if the spectral envelope is to be scaled over 6 octaves,
then cycles would be equal to 6, and base would be equal to 2;  if the
envelope is to span 7 perfect fifths, then set cycles to 7 and base to
3/2, etc.

There is a score example that uses shepenv in help file cmusic/ugs/illus.