Programming Question: How do you augment one matrix with another?

[Jsather]

This morning I saw your reply, Parisse, and also tried M1:=identity(2);[op(M1),op(M1)] and found that it works. So I tried other experiments and find [op(M1),op(M2)] augments M2 to M1 vertically in all cases, identically to concat(op(M1),op(M2)). As expected trn([op(trn(M1)),op(trn(M2))]) augments M2 to M1 horizontally.

I cannot explain why [op(M1),op(M2)] returned a list of two matrices on my virtual calculator yesterday and a vertical augmentation of M2 to M1 today.
I am certain of my observations yesterday but have been unable to return my virtual calculator to the state where [op(M1),op(M2)] fails, nor do I know how it got out of that state. I'll post If I figure it out.

That is good for CAS mode, but HP Prime programs work in Home mode. Is there any way to make M3:=[op(M1),op(M2)] work in an HP Prime program?