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void simulate_implicit_euler(lfVector *Xnew, lfVector *Vnew, lfVector *X, lfVector *V, lfVector *F, lfVector *olddV, fmatrix3x3 *dFdV, fmatrix3x3 *dFdX, float dt)
{
	lfVector *dFdXmV = create_lfvector(dFdV[0].vcount);
	lfVector *dV = create_lfvector(dFdV[0].vcount);
	
	// cp_bfmatrix(A, big_I);
	initdiag_bfmatrix(A, I);
	
	subadd_bfmatrixS_bfmatrixS(A, dFdV, dt, dFdX, (dt*dt));   
	
	mul_bfmatrix_lfvector(dFdXmV, dFdX, V);
	
	add_lfvectorS_lfvectorS(B, F, dt, dFdXmV, (dt*dt));
	
	cg_filtered(dV, A, B); /* conjugate gradient algorithm to solve Ax=b */

	// advance positions
	add_lfvector_lfvector(Vnew, V, dV);
	add_lfvector_lfvectorS(Xnew, X, V, dt);
	
	del_lfvector(dFdXmV);	
	del_lfvector(dV);
}