* Allow a default value in attribute selection by writing

x.y.z or default

  (as originally proposed in
  https://mail.cs.uu.nl/pipermail/nix-dev/2009-September/002989.html).

  For instance, an expression like

    stdenv.lib.attrByPath ["features" "ckSched"] false args

  can now be written as

    args.features.ckSched or false

src/libexpr/lexer.l

@@ -96,6 +96,7 @@ let         { return LET; }
 in          { return IN; }
 rec         { return REC; }
 inherit     { return INHERIT; }
+or          { return OR_KW; }
 \.\.\.      { return ELLIPSIS; }
 
 \=\=        { return EQ; }