cautionary Key cancellation in brackets

[Jiri Zurek (Prague)]

I need to put the cancellation key signature (cautionary natural sign after
the F-major music block ended and changed into the G-major) into brackets.
Cheating from a snippet in the documentation, I learned how to put the key
signature into brackets using the lambda function:
\override Staff.KeySignature #'stencil = #(lambda (grob) (bracketify-stencil
(ly:key-signature-interface::print grob) Y 0.1 0.2 0.1)).
However, this function puts into the brackets only the normal key signature.
When the key signature changes during the piece, there is a cancellation key
signature printed before the next key signature. And I need to put into
brackets this cancellation key signature. Could you please help me to modify
the above mentioned function so, that it would put into the brackets not the
normal key signature but only the cautionary cancellation key signature?

Here is the example:

%begin lily
\version "2.13.18"
\score { 
    \new Voice = "cantus" { 
\clef "G_8" 
\override Staff.KeySignature #'stencil = #(lambda (grob) (bracketify-stencil
(ly:key-signature-interface::print grob) Y 0.1 0.2 0.1)) 
\key f \major 
bes4  
bes4  
\key g \major  
b4 
b4 
} 

\layout {
}
}
%end lily

Thank you,
Jiri Zurek
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