5.5 Mixins
(mixin (interface-expr ...) (interface-expr ...) class-clause ...)
Produces a mixin, which is a procedure that encapsulates a
class extension, leaving the superclass unspecified. Each time that a
mixin is applied to a specific superclass, it produces a new derived
class using the encapsulated extension.
The given class must implement interfaces produced by the first set of interface-exprs. The result of the procedure is a subclass of the given class that implements the interfaces produced by the second set of interface-exprs. The class-clauses are as for class*, to define the class extension encapsulated by the mixin.
Evaluation of a mixin form checks that the class-clauses are consistent with both sets of interface-exprs.