( D+!                         add the double number to address )

need roll need d@
: d+!                             ( d addr --- )
  dup                             ( copy addr)
  d@                              ( fetch double number at addr)
  5 roll                          ( rotate lo order 16 bits)
  5 roll                          ( rotate hi order 16 bits)
  d+                              ( add the two double numbers)
  rot                             ( rotate addr to top)
  d!                              ( store the result)
;