Before revision:  Indeed, we first show that b(X(t))=l  (t), with l  the unique optional solution of a representation problem in the spirit of Bank-El Karoui  (2004)  ;
After revision:  Indeed, we first show that b(X(t))=l (t), with l the unique optional solution of a representation problem in the spirit of Bank-El Karoui Ann. Probab. 32 (2004) ;