( ';' can't have its name right away because, when created, it
  is not an IMMEDIATE yet and will not be treated properly by
  xcomp. )
: _
    ['] EXIT ,
    R> DROP     ( exit : )
; IMMEDIATE
';' XCURRENT @ 4 - C!
':' X' _ 4 - C! ( give : its name )
'(' X' _ 4 - C!