Notation for appending 2 submatrices

Question

I have a matrix $M$ with $i$ rows and $(e+n)$ columns: $M_{i,e+n}$

I would like to express that $M_{i,e+n}$ is the result of appending $M_{i,e}$ and $M_{i,n}$

What is the algebraic notation to express this, i.e. the "append" operation??

Thanks