--- a/Notes.md	Sat Dec 23 23:01:28 2023 +0100
+++ b/Notes.md	Sat Dec 23 23:03:13 2023 +0100
@@ -331,3 +331,15 @@
 Could the implementation of LazyOuterProduct be simplified by making it a
 struct containing two or more LazyTensors? (using split_tuple in a similar way
 as TensorGrid)
+
+## Implementation of boundary_indices for more complex grids
+To represent boundaries of for example tet-elements we can use a type `IndexCollection` to index a grid function directly.
+
+```julia
+I = IndexCollection(...)
+v[I]
+```
+
+* This would impact how tensor grid works.
+* To make things homogenous maybe these index collections should be used for the more simple grids too.
+* The function `to_indices` from Base could be useful to implement for `IndexCollection`