I had just done a minimal (command line only) install of Stretch from
the first DVD of the complete set rather than using netinst to conserve
bandwidth. The install as usual was successful.
I wish to install a package from the repository - it's not on DVD1.
I added the line
deb http://ftp.us.debian.org/debian/ stretch main non-free contrib
to sources.list while commenting out the line referencing CDROM.
I attempted to run "apt-get update".
It *FAILED*
The message was the source I wanted was being ignored.
WHY?