Valid devices in yapp_mongo are (see also MONGO-87B.5users manual) numbers 1..6, -6..-1:
1. Retrographics 640 -1. Versatec (portrait)
2. DEC VT125 -2. Versatec (landscape)
3. Tektronix 4010 -3. Printronix (portrait)
4. Grinell 270 -4. Printronix (landscape)
5. HP 2648A -5. Postscript (portrait)
6. Sun Windows -6. Postscript (landscape)
-7. Postscript (square portrait) ***
Make sure you have set:
setenv MONGOPATH /usr/local/lib/mongo
setenv MONGOFILES $MONGOPATH/mongofiles.dat
or appropriate private ones, or use whatever your system manager has
provided for.