1 files changed, 3 insertions, 0 deletions
diff --git a/src/AST.hs b/src/AST.hs
index b8bee1b..a10f1ae 100644
--- a/src/AST.hs
+++ b/src/AST.hs
@@ -82,6 +82,9 @@ data Expr x env t where
   -- be backpropagated to; 'a' is the inactive part. The dual field of
   -- ECustom does not allow a derivative to be generated for 'a', and hence
   -- none is propagated.
+  -- No accumulators are allowed inside a, b and tape. This restriction is
+  -- currently not used very much, so could be relaxed in the future; be sure
+  -- to check this requirement whenever it is necessary for soundness!
   ECustom :: x t -> STy a -> STy b -> STy tape
           -> Expr x [b, a] t  -- ^ regular operation
           -> Expr x [D1 b, D1 a] (TPair (D1 t) tape)  -- ^ CHAD forward pass