SET

The value of the set special form is undefined, and it is an error to use a set form in a value requiring position. For example,

formula vobeyspaces :

!:}(set (p x y ...) val)

is conceptually equivalent

formula vobeyspaces :

!:} (lambda () ((setter p) x y ...val) (return))

where (return) invokes the calling continuation with no arguments. For more information on return see section . In version 3.0 set will continue to return the value being assigned to the location, but an error will be signalled in the future.