The sentence that should be added to summarize the passage is;
A summary should follow sequentially from the last information provided.
So we can deduce that after Lala had persuaded the raven to have a custard, he might have agreed to help but still trick Lala.
Since we are provided with a description of the Raven that reveals his attribute of trickery, we can conclude that the Raven might try to pull a fast one on Lala.
Lala does not trust the Raven because of his antecedents.
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