; define constants

(defconstant feet_per_meter 3.28)
(defconstant meter_per_foot (/ 1.0 feet_per_meter))
(defconstant feet_per_mile 5280.0)
(defconstant meters_per_kilometer 1000.0)
(defconstant miles_per_foot (/ 1.0 feet_per_mile))
(defconstant miles_per_kilometer (* miles_per_foot (* feet_per_meter meters_per_kilometer)))

; define a function
(defun cvt_feet_to_meters(feet)
  (setq meters (* feet meter_per_foot)))