# Seemingly extension of example in SciMLOperators doesn't work

**URL:** <https://discourse.julialang.org/t/seemingly-extension-of-example-in-scimloperators-doesnt-work/127241>\
**Category:** Specific Domains\
**Tags:** question, sciml, operator\
**Created:** [March 21, 2025, 7:28pm UTC](https://discourse.julialang.org/t/seemingly-extension-of-example-in-scimloperators-doesnt-work/127241 "2025-03-21T19:28:05Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![Veenty](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/veenty/32/50940_2.png) [@Veenty](https://discourse.julialang.org/u/Veenty)\
**Post date:** [March 21, 2025, 7:28pm UTC](https://discourse.julialang.org/t/seemingly-extension-of-example-in-scimloperators-doesnt-work/127241/1 "2025-03-21T19:28:05Z")

</div>

JUst following the tutorial I would like to be able to define operator L6, as follows

```julia

N = 4
f = (u, p, t) -> u .* u

M = MatrixOperator(rand(N, N))
D = DiagonalOperator(rand(N))
F = FunctionOperator(f, zeros(N), zeros(N))

L1 = 2M + 3F + LinearAlgebra.I + rand(N, N)
L2 = D * F * M'
L3 = kron(M, D, F)
L4 = M \ D
L5 = [M; D]' * [M F; F D] * [F; D]
L6 = [F; D] #generates a 2N x N operator

L6(rand(N), nothing, 0)

```

But the last line generates the error

```julia
ERROR: MethodError: objects of type Vector{SciMLOperators.AbstractSciMLOperator{Float64}} are not callable
Use square brackets [] for indexing an Array.
The object of type `Vector{SciMLOperators.AbstractSciMLOperator{Float64}}` exists, but no method is defined for this combination of argument types when trying to treat it as a callable object.

```

Any ideas on how I could handle this?

---

<div class="post-metadata">

**Author:** ![ChrisRackauckas](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/chrisrackauckas/32/77_2.png) [@ChrisRackauckas](https://discourse.julialang.org/u/ChrisRackauckas)\
**Post date:** [March 23, 2025, 6:46am UTC](https://discourse.julialang.org/t/seemingly-extension-of-example-in-scimloperators-doesnt-work/127241/2 "2025-03-23T06:46:10Z")

</div>

We’d need to overload the `cat` functions in order to do this. Can you open an issue?