Economics - quotas - mistake in my textbook?

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 accompanying text to the graph below says:

Domestic producers now supply 0Q1 and Q3Q4 tons of wheat at a price of Pquota. 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 0Q1 quantity for price Pw so they make profit of a and then they sell quantity Q3Q4 for price Pquota which gives area of the rectangle: c+i+j+d. I have no idea what that f is doing there. Can anyone explain?

Here's the image of the graph:
http://i.stack.imgur.com/BsMwE.png
Here's the same question on StackExchange if you prefer math font: http://economics.stackexchange.com/questions/6575/quotas-mistake-in-my-textbook