[exim] acl expansion

In the ${acl {FOO}} construction, does (the expansion of) FOO have to be
strictly an ACL name, or is an algorithm like the one in acl_smtp_rcpt =
FOO followed? In particular can FOO expand to "/etc/exim/foo.acl" ?

If it's not possible now, would a feature request (or a patch) be
accepted?

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