以為得計 He Is In My Belly
卻說那老魔吞了行者，以為得計，徑回本洞，眾妖迎問出戰之功。老魔道：「拿了一個來了。」二魔喜道：「哥哥拿的是誰？」老魔道：「是孫行者。」二魔道：「拿在何處？」却说那老魔吞了行者，以为得计，径回本洞，众妖迎问出战之功。老魔道：「拿了一个来了。」二魔喜道：「哥哥拿的是谁？」老魔道：「是孙行者。」二魔道：「拿在何处？」
逐句解 Line by line
Now the old fiend, having swallowed Wukong and reckoning he had managed it well, went straight back to his own cave, where the demons came out to ask after his success in the field.
以為 takes 為 at wéi — to take it that, to reckon — and the sealed chapters read it that way at both their sites. 得計 keeps 得 at the full dé: the plan came off.
The old fiend said, "I have brought one back."
Understated, and the understatement is the point. He has caught the Great Sage and he announces it as an item.
The second monster said happily, "Who is it that you caught, elder brother?"
The second chief, the elephant, has spoken exactly twice in this chapter — four characters after the door-knocking, and this. He is the least developed of the three and the book knows it.
The old fiend said, "Sun Wukong."
Four characters, and the whole hall now knows the afternoon has been a success.
The second monster said, "Where have you got him?"
The obvious question, and the answer is going to spoil the party.
The exchange is deliberately flat and slow — five short speeches to establish that a demon chief has come home victorious. The book is winding the spring.
以為得計 is the narrator's judgement rather than the demon's, and it does the work of a whole paragraph. He reckoned he had managed it. Four characters, and everything that follows is that reckoning coming apart.