Diagonal v. sparse matrices

[Jason Riedy]

And Jaroslav Hajek writes:
> What's wrong with it? Besides, in current tip you get
> P =
> Permutation Matrix
>    0   1   1
>    1   0   1
>    1   1   0

It should be
    1   0   0
    0   1   0
    0   0   1
to match traditional mathematical notation.

> What exactly are you trying to achieve? Remember that most sparse vs.
> diag ops return a full matrix for compatibility with matlab, and
> changing that would be a significant breach.

I don't care about Matlab(TM).  I don't use it.  They don't want
me as a user, personally or as a class; they're targeting a
different market now.

> That's what I did for the mx-op-defs and MArray-defs. I think there
> was some speed up, but still not what I was hoping for. So I guess C++
> is just slow, and we need to live with that.

I'll look into that.  I'd hope it'd break some of the dependency
chains.  It's the *re*building of everything that's killing me.

Jason