;;
;; The macro "and-." expands to a pair unless its second argument is #f
;; or (if a third argument is given) the third argument is #f
;;
  (define-syntax and-.
    (syntax-rules ()
      ((and-. x y) (and y (cons 'x  y)))
      ((and-. x y z) (and z (cons 'x  y)))))
;;
;; The macro "filter-true" filters all elements equal to #f from a list.
;;
  (define-syntax filter-true
    (syntax-rules ()
      ((filter-true lst) (filter (lambda(x) x) lst))))