hypirion

(defn lookup
  "Lookup a variable in the model.  Respects inversion of the variable if it is
  specified with a - symbol.  Returns nil if the variable is unset."
  [[first-char :as var] model]
  (if (= first-char \-)                         ; If the first character is '-'
    (if-let [[_ val] (find model (subs var 1))] ; test for existence
      (not val)                                 ; invert value if exists
      nil)                                      ; otherwise return nil
    (get model v)))