aux variable: n. a variable that occurs in the part of a lambda list
that was introduced by &aux. Unlike all other variables
introduced by a lambda-list, aux variables are not
parameters.
auxiliary method: n. a member of one of two sets of methods
(the set of primary methods is the other)
that form an exhaustive partition of the set of methods
on the method's generic function.
How these sets are determined is dependent on the method combination type;
see Section 7.6.2 Introduction to Methods.