Modify subrequest header

I am trying to use ngx_http_subrequest in my customize nginx module. I
can
see from the code that the subrequest share the same request header with
the
main request(sr->headers_in = r->headers_in). Is there a way to modify,
add
or delete request header for subrequest without affecting the request
header
of the main request?

I tried ngx_list_init(&sr->headers_in.headers and use ngx_list_push to
push
new header in. And it is giving me a runtime error. Can someone point me
to
the right direction?

Thanks

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