\paper { tagline = ##f }

#(define (key-alt-key-stencil p-1 p-2 scale-def bracket-style)
  (lambda (grob)
    (let* ((staff-space (ly:staff-symbol-staff-space grob))
           (th (ly:staff-symbol-line-thickness grob))
           (default-stil (ly:key-signature-interface::print grob))
           ;; a-minor and c-major don't have a visible KeySignature
           ;; TODO: add support for other scales: ionian, locrian etc
           (no-visible-key?
             (lambda (p)
               (or (and (= (ly:pitch-notename p) 5)
                        (= (ly:pitch-alteration p) 0)
                        (eq? scale-def minor))
                   (and (= (ly:pitch-notename p) 0)
                        (= (ly:pitch-alteration p) 0)
                        (eq? scale-def major)))))
           (stil-to-add
             (if (no-visible-key? p-2)
                 #f
                 (grob-interpret-markup grob 
                   #{
                     \markup
                       \score {
%% updaters remark: 
%% 2.22. warns for a loney KeySignature, thus we insert s1*1/100000.
%% Otherwise an assertion error happens.
                         { \key $p-2 $scale-def s1*1/100000 }
                         \layout {
                           \omit Staff.TimeSignature
                           \omit Staff.Clef
                           \override Staff.StaffSymbol.line-count = #0
                           indent = 0
                         }
                     }
                   #}))))
      (if stil-to-add
          (let* ((stil-to-add-x-ext (ly:stencil-extent stil-to-add X))
                 (stil-to-add-y-ext (ly:stencil-extent stil-to-add Y))
                 (new-key-sig-stil
                   (ly:make-stencil
                     (ly:stencil-expr stil-to-add)
                     ;; left bracket is too far away, trimmed a little
                     (cons
                        (+ (car stil-to-add-x-ext) (* 0.7 staff-space))
                        (cdr stil-to-add-x-ext))
                     ;; adjusting the top and bottom ending of the bracket
                     (cons 
                        (+ (car stil-to-add-y-ext) (* 0.2 staff-space)) 
                        (- (cdr stil-to-add-y-ext) (* 0.5 staff-space)))))
                 (bracketified-stil
                   (cond ((eq? bracket-style 'bracketified)
                          (apply bracketify-stencil 
                                 (list 
                                   ;; stil 
                                   new-key-sig-stil 
                                   ;; axis 
                                   Y 
                                   ;; thick 
                                   th 
                                   ;; protrusion 
                                   (* 2.5 th) 
                                   ;; padding
                                   th)))
                         ((eq? bracket-style 'parenthesized)
                          (apply parenthesize-stencil 
                                 (list 
                                   ;; stencil 
                                   new-key-sig-stil 
                                   ;; half-thickness 
                                   th 
                                   ;; width 
                                   (* 0.5 staff-space) 
                                   ;; angularity 
                                   (* 2.5 th) 
                                   ;; padding
                                   th)))
                         (else
                           (ly:warning "Unknown bracket-style: ~a, ignoring"
                                       bracket-style)
                           new-key-sig-stil))))
          (if (no-visible-key? p-1)
              bracketified-stil
              (ly:stencil-combine-at-edge 
                default-stil
                X
                RIGHT
                bracketified-stil
                staff-space)))
         default-stil))))

alternativeKey =
#(define-music-function (style p1 p2 scale)
     ((symbol? 'bracketified) ly:pitch? ly:pitch? number-pair-list?)
#{
  \override Staff.KeySignature.stencil =
   #(key-alt-key-stencil p1 p2 scale style)
  \key $p1 $scale
#})

%%%%%%%%%%%%%%
% EXAMPLES
%%%%%%%%%%%%%%
%%{
\relative c' {
	\alternativeKey c f \major
	c1
	\alternativeKey b c \major
	c
	\alternativeKey e ees \major
	c
	\alternativeKey a aes \major
	c
	\alternativeKey #'parenthesized d des \major
	c
	\alternativeKey g ges \major
	c
	\alternativeKey c ces \major
	c
	\alternativeKey ges g \major
	c
	\alternativeKey des d \major
	c
        \alternativeKey aes a \major
        c
	\alternativeKey ees e \major
	c
	\alternativeKey bes b \major
	c
	\alternativeKey f fis \major
	c
	\alternativeKey ces cis \minor
	c
}