(expand L:list FN:function): list expr
Expand is a convenient way to expand a certain type of macro. PSL supports
functions like plus which accept any number of arguments. The macro plus
is actually defined in terms of the binary operator plus2. For example,
FN should be a function which accepts two arguments, and L should be a list of values which
are acceptable to FN as arguments.
(robustexpand L: list FN: function EMPTYCASE: form): list expr
If the list L is empty then EMPTYCASE is evaluated, otherwise the result
of (expand L FN) is returned.