Crafting a filter for "to recipient R and only recipient R"

Howdy,

I've been trying to come up with a way to create a filter for mail where
my wife is the only recipient, but having trouble crafting it.  The aim
would be to craft a hook based on something like

  ~t [email protected] [magic here]

pulling in a different (disgustingly smoochy) .signature for her if
she's the only recipient, but if anybody else is in the To: or CC: or
BCC: just use the default signature.

Is there a way to do this that I've missed?

Thanks!

-tkc
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