  We obtain a prior $C^{1,1}$ estimates for some Hessian (quotient) equations
with positive Lipschitz right hand sides, through studying a twisted special
Lagrangian equation. The results imply the interior $C^{2,\alpha}$ regularity
for $C^0$ viscosity solutions to $\sigma_2=f^2(x)$ in dimension 3, with
positive Lipschitz $f(x)$.
