Dear Firedrakers,

I'm trying to update a vector function in my time loop

a.interpolate(Expression(["0.5*F0*cos(omega*t)*x[1]", "-0.5*F0*cos(omega*t)*x[0]"], F0=F0, omega=omega, t=t))

but if I use assign or interpolate, I get told the form does not have the respective property. 
How should I go about making this work, such that it'll change the value of a in my solver?

Thanks

will