diff --git a/HOperator.jl b/HOperator.jl
index 07efdff..10b0259 100644
--- a/HOperator.jl
+++ b/HOperator.jl
@@ -122,14 +122,8 @@ function LinearAlgebra.mul!(out::CuArray{Complex{T}}, H::HOperator{T}, v::CuArra
     return out_t.data
 end
 
-"Apply 'H' on 'v' and return the result using the 'cpu_tensor' backend"
-function (H::HOperator{T})(v::Array{Complex{T}})::Array{Complex{T}} where {T<:Float}
-    out = similar(v)
-    return mul!(out, H, v)
-end
-
-"Apply 'H' on 'v' and return the result using the 'gpu_cutensor' backend"
-function (H::HOperator{T})(v::CuArray{Complex{T}})::CuArray{Complex{T}} where {T<:Float}
+"Apply 'H' on 'v' and return the result"
+function (H::HOperator)(v)
     out = similar(v)
     return mul!(out, H, v)
 end
diff --git a/example.ipynb b/example.ipynb
index a8caef4..8b5103f 100644
--- a/example.ipynb
+++ b/example.ipynb
@@ -2,9 +2,23 @@
  "cells": [
   {
    "cell_type": "code",
-   "execution_count": null,
+   "execution_count": 5,
    "metadata": {},
-   "outputs": [],
+   "outputs": [
+    {
+     "ename": "LoadError",
+     "evalue": "LoadError: invalid redefinition of constant cpu_tensor\nin expression starting at c:\\Users\\yapan\\DVR.jl\\HOperator.jl:4",
+     "output_type": "error",
+     "traceback": [
+      "LoadError: invalid redefinition of constant cpu_tensor\n",
+      "in expression starting at c:\\Users\\yapan\\DVR.jl\\HOperator.jl:4\n",
+      "\n",
+      "Stacktrace:\n",
+      " [1] top-level scope\n",
+      "   @ Enums.jl:204"
+     ]
+    }
+   ],
    "source": [
     "# prerequisite packages: KrylovKit, TensorOperations, LinearAlgebra, CUDA#tb/cutensor, Plots\n",
     "include(\"HOperator.jl\")\n",
@@ -14,9 +28,18 @@
   },
   {
    "cell_type": "code",
-   "execution_count": null,
+   "execution_count": 6,
    "metadata": {},
-   "outputs": [],
+   "outputs": [
+    {
+     "name": "stdout",
+     "output_type": "stream",
+     "text": [
+      "  4.377701 seconds (5.09 M allocations: 764.743 MiB, 6.09% gc time, 71.04% compilation time: 99% of which was recompilation)\n",
+      "114 operations : ComplexF32[-7.6208663f0 + 0.0f0im, -3.551723f0 + 0.0f0im, -3.5371912f0 + 0.0f0im, -3.5240355f0 + 0.0f0im, -3.5159583f0 + 0.0f0im, -3.4865863f0 + 0.0f0im, -3.1896422f0 + 0.0f0im, -2.9661055f0 + 0.0f0im]\n"
+     ]
+    }
+   ],
    "source": [
     "V_gauss(r2) =\n",
     "    -4 * exp(-r2 / 4)\n",
@@ -37,7 +60,7 @@
   },
   {
    "cell_type": "code",
-   "execution_count": null,
+   "execution_count": 7,
    "metadata": {},
    "outputs": [],
    "source": [