--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/StencilIndex.jl	Tue Jan 29 14:32:28 2019 +0100
@@ -0,0 +1,43 @@
+abstract type StencilIndex end
+
+function Base.getindex(si::StencilIndex, i::Int)
+    return si.index[i]
+end
+
+struct LowerClosureIndex <: StencilIndex
+    index::CartesianIndex
+end
+
+struct UpperClosureIndex  <: StencilIndex
+    index::CartesianIndex
+end
+
+struct InteriorIndex  <: StencilIndex
+    index::CartesianIndex
+end
+
+# TODO: This should take a Stencil or DiffOp so that we can extract all the
+# indices in the closures.
+# TODO: Where to place this function?
+function stencilindices(grid::Grid.EquidistantGrid)
+    lowerclosure = Vector{LowerClosureIndex}(undef, 0)
+    upperclosure = Vector{UpperClosureIndex}(undef, 0)
+    interior = Vector{InteriorIndex}(undef, 0)
+    # TODO: Fix such that the indices of the entire closure width is included.
+    islower = x -> (x == 1)
+    isupper = x -> (x in grid.numberOfPointsPerDim)
+    ci = CartesianIndices(grid.numberOfPointsPerDim)
+    for i ∈ ci
+        I = Tuple(i)
+        if any(islower, I)
+            push!(lowerclosure, LowerClosureIndex(i))
+            # TODO: Corner points should be in both Lower and Upper?
+            #       Should they have a separate type?
+        elseif any(isupper, I)
+            push!(upperclosure, UpperClosureIndex(i))
+        else
+            push!(interior, InteriorIndex(i))
+        end
+    end
+    return lowerclosure, upperclosure, interior
+end