 We find that epistasis is the nonlinearity of the Green function, which corresponds to a sum over multiple scattering paths passing through the localized perturbations .
<clarity>  We find that epistasis is the nonlinearity of the Green function, which corresponds to a sum over multiple scattering paths passing through the localized perturbations .
 We find that epistasis is the nonlinearity of the Green function, which corresponds to a sum over multiple scattering paths .