def solve(self, lam):
        '''Solves the GFL for a fixed value of lambda.'''
        s = weighted_graphtf(self.nnodes, self.y, self.weights, lam,
                             self.Dk.shape[0], self.Dk.shape[1], self.Dk.nnz,
                             self.Dk.row.astype('int32'), self.Dk.col.astype('int32'), self.Dk.data.astype('double'),
                             self.maxsteps, self.converge,
                             self.beta, self.u)
        self.steps.append(s)
        return self.beta
Solves the GFL for a fixed value of lambda.