#+property: header-args :var foo=1
#+property: header-args+ :var bar=2
#+begin_src emacs-lisp
(+ foo bar)
#+end_src
* overwriting a file-wide property
:PROPERTIES:
:header-args: :var foo=7
:END:
foo
* appending to a file-wide property
:header-args+: :var baz=3
(+ foo bar baz)