Macro concat 

macro_rules! concat {
    () => { ... };
    ($([$($v:expr),* $(,)?] ),* $(,)?) => { ... };
}

concatenates the matrices in each row horizontally, then concatenates the results vertically

concat![[a0, a1, a2], [b1, b2]] results in the matrix

[a0 | a1 | a2][b0 | b1]