diff --git a/_index.toml b/_index.toml
index 9a7dfd1..c17628f 100644
--- a/_index.toml
+++ b/_index.toml
@@ -1,10 +1,25 @@
+# TODO: proper documentation
+
+# These are paths to subdirectories,
+# each should contain another _index.toml.
+#
+# textures defined in subindices get a prefix,
+# currently "{path}::"
+#
+# TODO: rename "texture" to "sprite"
+# TODO: check all failure modes
+# - this should always be a relative path
+# - should we allow slashes?
+# - error if there is no _index?
+# - set different prefix?
 include = ["planet", "star"]
 
+
+# Only the root _index.toml may contain config.
 [config]
 error = "error"
 starfield = "starfield"
 
-
 [[texture]]
 name = "error"
 path = "error.png"