caml_invalid_argument(str) is no return and does not free it's 
argument. 
So calling it with a string constructed dynamically will mean it'll never 
get freed.
I could construct an ocaml string and pass that to caml_raise_with_arg,
but I don't seem to be able to get to the caml_exn_Invalid_argument from
c. Am I missing something?
Immanuel