@testset "delta" begin
    A = rand(10, 5)
    B = rand(20, 5)
    d = KernelFunctions.Delta()
    @test pairwise(d, A; dims=1) == Matrix(I, size(A, 1), size(A, 1))
    @test pairwise(d, A, B; dims=1) == zeros(size(A, 1), size(B, 1))
    @test d(1, 2) == 0
    @test d(1, 1) == 1
    @test_throws DimensionMismatch d(rand(3), rand(4))
end
