Is ly:music? magic?

[Jérôme Plût]

Given the following file:

-------

#(begin
(define ((K x) y) x)
(define testA (define-music-function (top location x) (ly:music?)
  (make-music 'SequentialMusic 'void #t)))

; now we try to be clever
(define testB (define-music-function (top location x) ((K #t))
  (make-music 'SequentialMusic 'void #t)))
)

\testA a     % ok
\testB #'foo % ok
\testB a     % fail

------

The file fails to compile only on the third line, \testB a . So
obviously, using ly:music? as a type predicate accepts more inputs than
the always-true predicate. The only explanation would be that the
parser knows that it is expecting a music expression; in other words,
(ly:music?) is not only a predicate.

By the way, (define test1 ly:music?) works as ly:music? in the example
above, whereas (define (test2 x) (ly:music? x)) does not.

So: is there a way to define a polymorphic music function (as testB
should be in the above example)?

--
        Jérôme Plût