LIB "difform.lib";
ring R = 0,(x,y,z),ds;
diffAlgebra();
==> // The differential algebra Omega_R was constructed and the differential \
   forms dDx, dDy, dDz, dx, dy, dz are available.
/////////////////////////////////////
// Negation of a differential form //
/////////////////////////////////////
difform df = 13*dx*dy + 2*dy*dz - 6*dx*dy*dz - 3;
-df;
==> 3+(-13)*dx*dy+(-2)*dy*dz+6*dx*dy*dz
==> 
kill Omega_R,df,dx,dy,dz;