Double transpose when in tags within music function

[Ole V . Villumsen]

> I'm not top posting.

Expected behaviour of the example below: engrave a d (transpose the c to d).
Actual behaviour: engraved an f. Seems to have done both of the
transpositions described in the two different tags in the music function
(transposed c to d, then to f, or the other way around).

\version "2.18.0"

myTranspose =
  #(define-music-function(parser location theMusic) (ly:music?)
     #{
        \tag #'cd { \transpose c d #theMusic }
        \tag #'df { \transpose d f #theMusic }
     #})

{ \keepWithTag #'cd { \myTranspose { c'1 } } }

The problem seems to be only when the tags are inside a music function (if
this is not supposed to work, an appropriate error message should be given).
The problem seems to be only with transposition; other music I put inside
the tags gets filtered as expected.