Does there exist a fairly standard notation for horizontal and vertical juxtaposition operations on matrices? (Vertical juxatposition can be called "stacking.")
For example, juxtaposing horizontally a matrix of the size $m\times n_1$ with a matrix of the size $m\times n_2$, one obtains a matrix of the size $m\times(n_1 + n_2)$. Stacking vertically a matrix $m_1\times n$ with a matrix $m_2\times n$, one obtains a matrix $(m_1 + m_2)\times n$.
I've seen the notation $$ A = [a_1|a_2|\dotsb|a_n] $$ for a matrix $A$ with columns $a_1,a_2,\dotsc,a_n$. It looks a bit ad hoc, unless we define $|$ as the horizontal juxtaposition operation that can be applied to any pair of matrices with the same number of rows. A notation for vertical juxtaposition is also needed.
Such notation would be quite useful for writing matrices defined by blocks or decomposing matrices into blocks (into rows or columns in particular).