 Lifting a subspace through a Freiman model

Under injective linear modelling of a small sumset, every $w\in W$ has a unique lift $\sigma (w)\in 4B$, because differences of two lifts lie in $8B\subseteq 12B$. For $w_1,w_2\in W$, the discrepancy $\sigma (w_1)+\sigma (w_2)+\sigma (w_1+w_2)$ lies in $12B\cap \ker L$, so it is zero. Thus $\sigma$ is a linear map and its image is a vector subspace of the same size as $W$. This verifies the additive lifting step rather than assuming that a preimage chosen from a sumset is automatically a subspace.