3.4.1.5 Specifiers for &aux variables

These are not really parameters. If the lambda list keyword &aux is present, all specifiers after it are auxiliary variable specifiers. After all parameter specifiers have been processed, the auxiliary variable specifiers (those following &aux) are processed from left to right. For each one, init-form is evaluated and var is bound to that value (or to nil if no init-form was specified). &aux variable processing is analogous to let* processing.

 (lambda (x y &aux (a (car x)) (b 2) c) (list x y a b c))
    ==  (lambda (x y) (let* ((a (car x)) (b 2) c) (list x y a b c)))