@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
