So, I'm using a library that calls Arg.parse to build up it's inputs in
another data structure. I don't want to rewrite any code, and I'm consuming
the library in a different way than how the binary consumes it, which is
just to feed it input from the command line.

Not wanting to replicate code in two locations, I chose to use
Arg.parse_argv, and supplied it with an array that would normally be on the
command line. For some reason, supplying the exact same speclist as what
was used in the original statement that works as a command line tool
doesn't result in the parameters being parsed.

command line tool:
Arg.parse speclist anon usage;
get_program () (* consumes the mutable list that speclist modified *)

(* speclist modifies a mutable list used to hold arguments *)

my code that consumes speclist as a library:

Arg.parse_argv custom_arguments speclist anon usage
get_program () (* exact same function as above, exact same variables to
Arg.parse* *)

For some reason, Arg.parse_argv seems to complete, but when get_program
proceeds, it sees the mutable argument list as being empty. Why?