[Xy-pic] loop in xy-pic

[Thierry Coquand]

 I have just started to use xy-pic for drawing some simple
finite-state automata

 The following example was simple to write and produce what
is expected

\newcommand\putc[1]{*++[o][F-]{#1}}


$ 
  \xymatrix{
     \putc{A} \ar@/_1pc/[rr]_{\lambda} \ar@/_1pc/[dd]_{\alpha} & & \putc{B} \ar@/_1pc/[ll]_{\mu} \\
                                                                         \\
     \putc{C} \ar@/_1pc/[rr]_{\epsilon} & & *++[o][F=]{D}  \ar@/_1pc/[uu]_{\gamma} 
           }
$


 I would like now to add a loop from the edge D to itself. 
 The following change is not accepted


$ 
  \xymatrix{
     \putc{A} \ar@/_1pc/[rr]_{\lambda} \ar@/_1pc/[dd]_{\alpha} & & \putc{B} \ar@/_1pc/[ll]_{\mu} \\
                                                                         \\
     \putc{C} \ar@/_1pc/[rr]_{\epsilon} & & *++[o][F=]{D}  \ar@/_1pc/[uu]_{\gamma} \ar@(r,d)[]^{b}
           }
$

 Is there a simple way to modify the previous diagram with a
loop from D to itself?? 

 Thanks in advance,

				Thierry Coquand