add default nix build target

flake.nix

@@ -32,17 +32,12 @@
           inherit buildInputs;
           shellHook = envSetup;
         };
-      } // (with pkgs; {
-        packages.documentation = build "documentation.pdf";
-        packages.theses = stdenv.mkDerivation {
-          name = name + "-theses";
+      } // (
+        with pkgs;
+        let
+          merge = buildInputs: name: stdenv.mkDerivation {
+            inherit name buildInputs;
             dontUnpack = true;
-          buildInputs = builtins.map (v: build v) [
-            "bp_cs.pdf" "bp_en.pdf"
-            "dp_cs.pdf" "dp_en.pdf"
-            "prj_cs.pdf" "prj_en.pdf"
-            "sp_cs.pdf" "sp_en.pdf"
-          ];
             installPhase = ''
               mkdir $out
               for input in $buildInputs
@@ -51,6 +46,19 @@
               done
             '';
           };
-      })
+          documentation = build "documentation.pdf";
+          theses = merge (builtins.map (v: build v) [
+              "bp_cs.pdf" "bp_en.pdf"
+              "dp_cs.pdf" "dp_en.pdf"
+              "prj_cs.pdf" "prj_en.pdf"
+              "sp_cs.pdf" "sp_en.pdf"
+          ]) (name + "-" + "theses");
+        in
+        {
+          packages.documentation = documentation;
+          packages.theses = theses;
+          packages.default = merge [documentation theses] name;
+        }
+      )
     );
 }