add describe function

tix/describe.nix

@@ -0,0 +1,8 @@
+name:
+map ({
+    msg,
+    success,
+    actual ? null,
+    expected ? null,
+  } @ result:
+    result // {msg = "${name}: ${msg}";})

tix/test.nix

@@ -1,14 +1,14 @@
 path: let
   it = import ./it.nix;
+  describe = import ./describe.nix;
   suite = import path;
 in
   builtins.trace ("testing " + builtins.baseNameOf path)
   (
-   if !builtins.isFunction suite
-     then throw "A test suite should be in the form { it }: [ <tests> ]."
-   else
-   {
-   inherit path;
-   results = suite {inherit it;};
-   }
+    if !builtins.isFunction suite
+    then throw "A test suite should be in the form { it }: [ <tests> ]."
+    else {
+      inherit path;
+      results = suite {inherit it describe;};
+    }
   )