(defmacro lc (&rest body)
  "Remove indentation.

- var := exp       - Bind exp to var.
- ! exp            - Eval exp.
- (when p exp)     - Branch when p.
- (unless p exp)   - Branch when (not p).
CPS helpers:
- arg-list :<> exp - Turn rest into a lambda bound to <>. Use in exp.
- :<> exp          - A shorthand for (&rest _) :<> exp."
  (let* ((b body)
         (fs '())
         (res))
    (while (not res)
      (pcase b
        ;; var := exp
        (`(,(and var (pred symbolp)) := ,exp . ,rest)
         (-> (lambda (body) `(let ((,var ,exp))
                               ,body))
             (push fs))
         (setq b rest))
        ;; lambda-list :<> exp
        (`(,arglist :<> ,exp . ,rest)
         (-> (lambda (body) `(let ((<> (lambda ,arglist
                                         ,body)))
                               ,exp))
             (push fs))
         (setq b rest))
        ;; :<>! exp
        (`(:<>! ,exp . ,rest)
         (-> (lambda (body) `(let ((<> (lambda (&rest ,(gensym))
                                         ,body)))
                               ,exp))
             (push fs))
         (setq b rest))
        ;; (when pred body ...)
        (`((when ,p . ,then) . ,else-rest)
         (-> (lambda (else) `(if ,p
                                 (progn ,@then)
                               ,else))
             (push fs))
         (setq b else-rest))
        ;; (unless pred body ...)
        (`((unless ,p . ,then) . ,else-rest)
         (-> (lambda (else) `(if (not ,p)
                                 (progn ,@then)
                               ,else))
             (push fs))
         (setq b else-rest))
        ;; ! exp
        (`(! ,exp . ,rest)
         (-> (lambda (body) `(progn ,exp ,body))
             (push fs))
         (setq b rest))
        ;; (else body ...)
        (`((else . ,rest))
         (setq res (-reduce-r-from #'funcall `(progn ,@rest) (reverse fs))))
        ('() (error "lc: Missing else"))
        (f (error "lc: Unrecognized form %S" f))))
    res))