25.  Matrices

      It is also possible to make matrices. For example, to make a neat array like

you have to type matrix {
ccol { x sub i above y sub i }
ccol { x sup 2 above y sup 2 }
}
This produces a matrix with two centered columns. The elements of the columns are then listed just as for a pile, each element separated by the word above. You can also use lcol or rcol to left or right adjust columns. Each column can be separately adjusted, and there can be as many columns as you like.

      The reason for using a matrix instead of two adjacent piles, by the way, is that if the elements of the piles don't all have the same height, they won't line up properly. A matrix forces them to line up, because it looks at the entire structure before deciding what spacing to use.

      A word of warning about matrices _ each column must have the same number of elements in it. The world will end if you get this wrong.