0-extent matrices?

[Thomas Lumley]

Implementing 0-extent matrices turns out to be easy -- it's almost
entirely a matter of removing the tests for 0.  There seems to be only one
thing that may require special-case handling, but I'm not sure what the
Right Thing to do is.

If m is a 0x4 matrix, say, what should 2*m, 2+m and so on return?

Since m+m is a 0x4 matrix but 2*as.vector(m) is NA we can't have complete
consistency.  My preference is for these operations to happen elementwise
-- ie m+m is a 0x4 matrix.  Comments?

Thomas Lumley
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