function boundary_restriction(grid::EquidistantGrid{2,T}, closureStencil::Stencil{T,M}, boundary::CartesianBoundary) where {T,M}
    r = region(boundary)
    d = dim(boundary)
    d_orth = 3-d # orthogonal dimension
    e = BoundaryRestriction(restrict(grid, d), closureStencil, r)
    I = IdentityMapping(size(restrict(g,d_orth)))
    if r == Lower
        return e⊗I
    elseif r == Upper
        return I⊗e
    else
        # throw error
    end
end
export boundary_restriction

"""
    BoundaryRestriction{T,N,R} <: TensorMapping{T,1,1}

Implements the boundary operator `e` as a TensorMapping
"""
struct BoundaryRestriction{T,M,R<:Region} <: TensorMapping{T,1,1}
    stencil::Stencil{T,M}
    size::NTuple{1,Int}
end
export BoundaryRestriction

function BoundaryRestriction(grid::EquidistantGrid{1,T}, closureStencil::Stencil{T,M}, region::Region) where {T,M}
    return BoundaryRestriction{T,M,typeof(region)}(stencil,size(grid))
end

LazyTensors.range_size(e::BoundaryRestriction) = (1,)
LazyTensors.domain_size(e::BoundaryRestriction) = e.size

" Restricts a grid function v on a grid of size m to the scalar element v[1]"
function LazyTensors.apply(e::BoundaryRestriction{T,M,Lower}, v::AbstractVector{T}, i::Index{Lower}) where {T,M}
    @boundscheck if Int(i)!=1
        throw(BoundsError())
    end
    apply_stencil(e.stencil,v,Int(i))
end

" Restricts a grid function v on a grid of size m to the scalar element v[m]"
function LazyTensors.apply(e::BoundaryRestriction{T,M,Upper}, v::AbstractVector{T}, i::Index{Upper}) where {T,M}
    @boundscheck if Int(i) != e.size[1]
        throw(BoundsError())
    end
    apply_stencil_backwards(e.stencil,v,Int(i))
end

" Transpose of a restriction is an inflation or prolongation.
  Inflates the scalar (1-element) vector to a vector of size of the grid"
function LazyTensors.apply_transpose(e::BoundaryRestriction{T,M,Lower}, v::AbstractVector{T}, i) where {T,M}
    @boundscheck if !(0 < Int(i) <= e.size[1])
        throw(BoundsError())
    end
    return e.stencil[Int(i)-1]*v[1]
end