(defun read-thing (string)
  (%read-thing string t))

(defgeneric %read-thing (string class)
  (:method-combination or))

(defmethod compute-applicable-methods ((generic-function (eql #'read-set)) args)
  (m:generic-function-methods #'read-set))

(defmethod %read-thing ((string string) (class (eql :foo)))
  (when (string= string "foo") :foo))

(defmethod %read-thing ((string string) (class (eql :bar)))
  (when (string= string "bar") :bar))

(defmethod %read-thing ((string string) (class (eql :baz)))
  (when (string= string "baz") :baz))

(defun test-me ()
  (list (read-thing "foo")
        (read-thing "bar")
        (read-thing "baz")))