24.  Piles

      There is a general facility for making vertical piles of things; it comes in several flavors. For example: A ~=~ left [
pile { a above b above c }
~~ pile { x above y above z }
right ]
will make

The elements of the pile (there can be as many as you want) are centered one above another, at the right height for most purposes. The keyword above is used to separate the pieces; braces are used around the entire list. The elements of a pile can be as complicated as needed, even containing more piles.

      Three other forms of pile exist: lpile makes a pile with the elements left-justified; rpile makes a right-justified pile; and cpile makes a centered pile, just like pile. The vertical spacing between the pieces is somewhat larger for l-, r- and cpiles than it is for ordinary piles. roman sign (x)~=~
left {
lpile {1 above 0 above -1}
~~ lpile
{if~x>0 above if~x=0 above if~x<0}
makes

Notice the left brace without a matching right one.