Inability to delete Discussion via dxl

With line 7 commented out, the following code will successfully print the Discussion summary for each discussion of a given object. My problem is that if line 7 is not commented, checkDXL raises 'incorrect arguments for function (delete)', which has me scratching my head, as the 9.2 dxl reference manual shows:
Declaration
string delete (Discussion& d)
 

Module m = current
Object o = current
string sResult
Discussion disc = null
for disc in o do {
  print "Discussion summary ; " disc.summary "\n"
//  sResult = delete(disc)
}

I'm obviously missing something. Hope someone can offer an answer. Would also appreciate if someone can explain the & in Discussion&