Lists can be used to represent sets. They can also be used to represent trees:

[ plus, a, b ]. [ plus, [times,a,b], [times,c,d] ].

Above, we use them to represent expressions built up from the operators
`plus`

and `times`

. If these operators are commutative, it is
often convenient to give them more than two arguments:

[ plus, a, b, c, d ].instead of

[ plus, [ plus, a, b ], [ plus, c, d ] ].

Many algebra-manipulation programs use this method.

Mon Jul 17 22:27:41 BST 1995