Impact of Import Quota onto Revenues I'm learning about quotas from my textbook but I've found one thing that doesn't make sense to me at all and I think it may be a mistake in the book.
The context: $P_w$ is the price of wheat on the world market. So far the domestic government has allowed free trade but now they set an import quota of $Q_3 - Q_1$.



The text accompanying the graph says:


  Domestic producers now supply $0Q_1$ and $Q_3Q_4$ tons of wheat at a
  price of $P_{quota}$. Their revenue rises from $a$ to $a+c+d+f+i+j$.


What I can't understand is why they included $f$ in the last sentence. In my opinion their revenue rises from $a$ to $a+c+d+i+j$. Domestic producers first sell $0Q_1$ quantity for price $P_w$ so they make profit of $a$ and then they sell quantity $Q_3Q4$ for price $P_{quota}$ which gives area of the rectangle: $c+i+j+d$.  I have no idea what that $f$ is doing there. Can anyone explain?
There is no mistake. The solution: it is assumed that there is only one price at the domestic wheat market. Hence domestic producers will not sell at price $P_w$ and price $P_{Quota}$ as well. This makes sense: Suppose you are a domestic producer and you are aware that the price that results from the quota system is $P_{Quota}$. Knowing this you would not sell your goods for the lower $P_w$.