Several customers have requested the ability to 'proof' the result of
overprinted DeviceN and Separation spaces on CMYK devices when the spot ink(s)
are not being produced.

The PostScript Language Reference Manual, PDF Reference and Tech Note 5145 state
that overprint (which is defined as device-dependent) is only honoured when the
relevant inks are being produced, thus DeviceN and Separation spaces which use
their alternate spaces should not overprint. However, there are conditions under
which it can be very useful to view overprints, especially when special inks
such as white, or varnish are used.

Currently Ghostscript will overprint DeviceN spaces when printing to CMYK, if
both overprint and overprintmode are true. This was introduced with revision
7858 (issue #689171), but is actually incorrect behaviour. This should be
restored to follow the old behaviour, and the action of /Separation spaces, 
when the new feature is implemented.

For more information on overprinting and spot inks see the PLRM 3rd edition p247
(Section 4.8.5), Tech Note 5044 (page 13, setoverprint) and Tech Note 5145,
especially :

"When overprintmode and overprint are both true, a tint value of 0.0 for a 
source color component leaves the corresponding component of the previously 
painted color unchanged. This behavior is called “nonzero overprint.

Nonzero overprint mode applies only to painting operations that use the current 
color in the graphics state when the current color space is Device CMYK. It does 
not apply to the painting of images or to any colors that are the result of a 
computation, such as those in a shading pattern or those converted from some 
other color space."