  The chiral space of local fields in Sine-Gordon or the SU(2)-invariant
Thirring model is studied as a module over the commutative algebra D of local
integrals of motion. Using the recent construction of form factors by means of
quantum affine algebra at root of unity due to Feigin et al. we construct a
D-free resolution of the space of local fields. In general the cohomologies of
the de Rham type complex associated with the space of local fields are
determined and shown to be the irreducible representations of the symplectic
group Sp(2\infty). Babelon-Bernard-Smirnov's description of the space of local
fields automatically incorporated in this framework.
