Variance in equilibrium equations

Hello,

I wanted to ask is it possible to have an equilibrium equation of the form

f(E_t(R_t+1)-R_t+1)Q_t S_t + f(E_t(S_t+1)-S_t+1) *(Q_t S_t-N_t)=N_t (non-loglinearized version)

where all variables are endogenous and f is arbitrary function. For example, if f(x)=x^2, I would get variance.

Thank you.

See [Variance in equilibrium equations)