foo.markdown

(defmacro serapeum.exporting:defclass (name supers &body (slots . options))
  ...)

is equivalent to:

(defmacro serapeum.exporting:defclass (name supers &rest more-stuff)
  (destructuring-bind (slots . options) more-stuff)
    ...))

which is equivalent to:

(defmacro serapeum.exporting:defclass (name supers &rest more-stuff)
  (destructuring-bind (slots &rest options) more-stuff)
    ...))

They use &body instead of &rest as an indentation hint.

They use the dotted-list form to avoid having to do destructuring-bind, and also because it provides a nicer documentation experience when your editor shows your the lambda list.