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The number of UNIX installations has grown to 10, with more expected.

--The Unix Programmer's Manual, 2nd Edition, June 1972

--The Unix Programmer's Manual, 2nd Edition, June 1972

\begin{bmatrix} 1 & 0 & \ldots & 0 \\
0 & 1 & 0 & \vdots \\
\vdots & 0 & \ddots & 0\\
0 & \ldots & 0 & 1_{n}
\end{bmatrix}