Copy Tree and Substitute

It is easy to write a function to copy a binary tree:

(defn copy-tree [form]
  (if (cons? form)
      (cons (copy-tree (first form))
            (copy-tree (rest form)))
      form) )

Why make an exact copy of a tree that we already have? Well, if we modify copy-tree slightly, we can make a copy with a substitution:

; substitute new for old in form

(defn subst [new old form]
  (if (cons? form)
      (cons (subst new old (first form))
            (subst new old (rest form)))
      (if (= form old)
          new
          form) ) )