Using a custom TeX .fmt file is Emacs/Auctex

Question

I am attempting to use a plain TeX format, say, foo.fmt (which is findable with kpsewhich), with Emacs/AUCTeX (actually Aquamacs with built-in AUCTeX). I'm unable to get AUCTeX to recognize the custom format. In TeXShop (a Mac TeX frontend) I would simply say at the top of the file:

% !TS-program=foo

In Emacs, I have tried various "Local Variables" such as

%%% Local Variables:
%%% TeX-engine: foo
%%% End:

followed by C-c C-n. C-c C-c returns an error indicating that the .fmt file has not been used. What am I missing?

Answer 1

I can't test this without your foo.fmt file, but I think you need to set this using the variable TeX-command-extra-options:

%%% Local Variables:
%%% TeX-command-extra-options: "--fmt=foo"
%%% End:

I'm not sure of the syntax, maybe it should be --fmt=foo.fmt instead.