For background, we start with an overview of the matrix command:
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Result
matrix { a # b ## c # d }
Rows are separated by two #'s and entries within each row are separated by one #.
The first problem people have with matrices is that brackets do not “scale" with the matrix:
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Result
( matrix { a # b ## c # d } )
()
OOoMath provides “scalable" brackets. That is, the brackets grow in size to match the size of their contents. Use the commands left( and right) to make scalable brackets.