--- a/Notes.md	Wed Nov 29 09:04:12 2023 +0100
+++ b/Notes.md	Wed Nov 29 09:14:06 2023 +0100
@@ -313,3 +313,14 @@
 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` which should be able to be used 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.