Alternating transset shortcut

[Andrew Conkling]

With transset, if you run it with no number parameter, it alternates
between making a window (semi-)transparent and completely opaque. 
However, the default setting is 0.75; I'd like to use 0.4.  So, two
questions:
Is there a way to make an alternating shortcut in my WM preferences that
would alternate between 0.4 and 1?
If not, does anyone know how to configure transset to use a different
number by default?  (There's no manpage at all.)

Thanks,
Andrew