Found a prob on GNU PROLOG

[Jean Michel LECONTE]

I'm using GNU Prolog 1.2.1 so i didn't try what follows on gnu prolog 1.2.6

I start gprolog interpreter

i ask for the goal U = o(U).
i have that (i switch the trace on) :

| ?- U = o(U).
      1    1  Call: _15=o(_15) ?
      1    1  Exit: 
o(o(o(o(o(o(o(o(o(...)))))))))=o(o(o(o(o(o(o(o(o(...))))))))) ?
 
                                                                            
and after it waits a long time and i have to use Ctrl-C to stop it


now i ask :

| ?- U = o(U),fail.
      1    1  Call: _15=o(_15) ?
      1    1  Exit: 
o(o(o(o(o(o(o(o(o(...)))))))))=o(o(o(o(o(o(o(o(o(...))))))))) ?
 
no  


Why prolog for my first goal doesn't answer me yes and why it waits a big 
time ???