anchor.diff:  
   (v-defspecial %merge-env (env-a new-env)
     :args-valid t
  -  :return  
  +  :return
     (let ((dummy-code-obj (make-instance 'code :current-line "")))
       (values dummy-code-obj (merge-env env-a new-env))))
   
  
positive.code:          (arg-glsl-names (loop :for (name) :in raw-args :collect
                              (safe-glsl-name-string (free-name name))))
           (body-obj (varjo->glsl `(,(if mainp 'progn '%clean-env-block)
                                     (%env-multi-var-declare ,args nil ,arg-glsl-names)
                                     ,@body) env))
           (glsl-name (safe-glsl-name-string (if mainp name (free-name name))))
           (primary-return (first (returns body-obj)))
  
negative.code:   (if (consp (first var-form))
        (error 'for-loop-only-one-var)
        (multiple-value-bind (code new-env)
            (varjo->glsl `(%glsl-let ,var-form t) env)
          (let* ((var-string (subseq (first (to-block code)) 0 (1- (length (first (to-block code))))))
                 (decl-obj (varjo->glsl (second var-form) new-env))
                 (condition-obj (varjo->glsl condition new-env))
  