cat and varying

what's the story, does the cat opcode have a problem with varying strings?

d sMsg            s             80a   varying
d sMsg2           s            256a   varying
c                   eval      sMsg = 'abc'
c     sMsg          cat       'def':1       sMsg2

the resulting value in sMsg2 is blanks.  I want it to be 'abc def'.

I was hoping that the CAT opcode would work best when appending to a string:
d sWord           s             80a   varying
d sSentence       s           2000a   varying

** using CAT
c                   cat       sWord:1       sSentence

** is more efficient than using "+" ?
c                   eval      sSentence = sSentence + ' ' + sWord

will the v5r2 += operator be more efficient than + ?
   sSentence += sWord ;
   sSentence = sSentence + sWord ;


thanks,

Steve Richter