PostScript extension ps-setpagedevice with Xerox7835 printer trays

With LaserJet, trays PostScript instruction are
for Tray 1 : /MediaPosition 1
for Tray 2 : /MediaPosition 2

With Xerox :
for Tray 1 : (tray1) xerox$MediaInputTray
for Tray 2 : (tray2) xerox$MediaInputTray
Here is an extract of my IF file (Intermediate Format) for LaserJet :

PS file for LaserJet :
Page 1 :
%FOPBeginSetPageDevice
<<
/PageSize [595 842]
/MediaPosition 1
/ImagingBBox null
>> setpagedevice
%FOPEndSetPageDevice

Page 2 :
%FOPBeginSetPageDevice
<<
/PageSize [595 842]
/MediaPosition 2
/ImagingBBox null
>> setpagedevice
%FOPEndSetPageDevice

PS file for Xerox :

Page 1 :
%FOPBeginSetPageDevice
<<
/PageSize [595 842]
(tray1) xerox$MediaInputTray <===== correct/ImagingBBox null
>> setpagedevice
%FOPEndSetPageDevice

Page 2 :
%FOPBeginSetPageDevice
<<
/PageSize [595 842]
(tray1) xerox$MediaInputTray <===== incorrect

(tray2) xerox$MediaInputTray <===== only this line is correct

/ImagingBBox null
>> setpagedevice
%FOPEndSetPageDevice

I have analysed the fop source code.

In fact, Fop use a key/Value HashTable and all new key/value are cumulative.
With LaserJet

Page 1 : key=/MediaPosition and value=1 => Fop write in PostScript : “/MediaPosition 1”
Page 2 : key=/MediaPosition and value=2 => Fop write in PostScript : “/MediaPosition 2”

With Xerox :

Page 1 : key=(tray1) and value=xerox$MediaInputTray => Fop write in PostScript : “(tray1) xerox$MediaInputTray”
Page 2 : key=(tray2) and value=xerox$MediaInputTray => Fop write in PostScript all keys/values : “(tray1) xerox$MediaInputTray” + “(tray2) xerox$MediaInputTray”