[O] Concatenating header args

[Michael Welle]

Hello,

how can I concatenate header args? Let's assume I have an Org structure
like follows:

* foo
 :PROPERTIES:
 :header-args: :flags -Wall
 :END:
** bar
#+begin_src C :flags -lm
#+end_src

Now I want the code to be compiled with -Wall _and_ -lm.

Regards
hmw