beams and ties in user defined functions

[Paul Malcolm]

I have a question on what might be possible in a user defined function 

with "music" passed to such a function, especially with respect to 

how notes are arranged in the etc  in the output

Suppose I have a little function like this

MyPattern = #(define-music-function (parser location x y) (ly:music? ly:music?)

#{

  $x[ $y~  $y]

#})

And subsequently in my code I call 

\MyPattern c8 c16

It seems this wont work as the tie and beaming brackets cause errors. 

Perhaps they are not allowed in a function. 

Is there any basic way to make this work ? 

Perhaps I have misunderstood the ly:music variable

My understanding is that the music variable has pitch and duration information. 

Perhaps Lilypond sees the []~ as inferring/corrupting this information ?

Paul