  The Estabrook-Wahlquist prolongation method is applied to the (compact and
noncompact) continuous isotropic Heisenberg model in 1 + 1 dimensions. Using a
special realization (an algebra of the Kac-Moody type) of the arising
incomplete prolongation Lie algebra, a whole family of nonlinear field
equations containing the original Heisenberg system is generated.
