[exim] Adding Headers to Top

Hi there

In http://www.ietf.org/internet-drafts/draft-schlitt-spf-classic-02.txt

it says

The Received-SPF header is a trace field (see [RFC2822] section
3.6.7) and SHOULD be prepended to existing headers, above the
Received: header that is generated by the SMTP receiver. It MUST
appear above any other Received-SPF headers in the message.

but

headers_add and warns always adds it at the bottom

How to I add a header to the top?

Thanks
Craig