anchor.diff:  
           nix-env -f . \
               --show-trace --argstr system "$platform" \
  -            -qa --drv-path --system-filter \* --system 2>&1 >/dev/null | tee eval-warnings.log
  +            -qa --drv-path --system-filter \* --system \
  +            "''${opts[@]}" 2>&1 >/dev/null | tee eval-warnings.log
   
           if [ -s eval-warnings.log ]; then
               echo "Nixpkgs on $platform evaluated with warnings, aborting"
  
positive.code: 
          nix-env -f . \
              --show-trace --argstr system "$platform" \
              -qa --drv-path --system-filter \* --system --meta --xml > /dev/null
          stopNest
      done
  
  
negative.code:     export NIX_DB_DIR=$TMPDIR
      export NIX_STATE_DIR=$TMPDIR
      export NIX_PATH=nixpkgs=$TMPDIR/barf.nix
      opts=(--option build-users-group "")
      nix-store --init
  
      echo 'abort "Illegal use of <nixpkgs> in Nixpkgs."' > $TMPDIR/barf.nix
  