Would it be relevent to mention that in TeX tabular and matricies *are* implemented using the same basic command, namely \halign? Matricies construct math boxes using a template contianing math shifts (aka $ signs) and tabular uses a template without math shifts. Plain TeX's \cases uses a template which has the first field in $ signs in the first column and not in the second (and I suspect other math mode machinery, for example \vcenter).
A \halign rendering engine could be leveraged to do other fancier constructions as well if you felt so inclined but that is not something for the immediate future. I moderately frequently use \halign for proofs that other people might render with eqnarray, which gives inferior spacing, sometimes with a flush right or otherwise seperated extra columns for sayying things like (lemma 42). The latter is simply not possible with eqnarray. -- Duncan (-: "software industry, the: unique industry where selling substandard goods is legal and you can charge extra for fixing the problems."
