anchor.diff:  (is (parse-template-string "{{loop seq as i}}<li>item{{var i}}</li>{{endloop}}")
       (att-progn
        (att-output (att-string ""))
  -     (att-loop (att-variable 'seq)
  +     (att-loop (att-variable 'arrows.lexer.default::seq)
                            (att-progn (att-output (att-string "<li>item"))
  -                                    (att-output (att-variable 'i))
  +                                    (att-output (att-variable 'arrows.lexer.default::i))
                                       (att-output (att-string "</li>")))
  -                         (att-variable 'i))
  +                         (att-variable 'arrows.lexer.default::i))
        (att-output (att-string "")))
       "loop"
       :test #'att-equal)
  
positive.code: (is (parse-template-string "{{var sym}}")
      (att-progn
       (att-output (att-string ""))
       (att-output (att-variable 'sym))
       (att-output (att-string "")))
      "var"
      :test #'att-equal)
  
negative.code: 
  (is (parse-template-string "the content of foo is {{include \"foo\"}}")
      (att-progn
       (att-output
        (att-string "the content of foo is "))
       (att-progn
  