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Out of curiosity...
In ROTJ, it's only implied that Luke went back prior to the events in the film and completed training under Yoda - never stated, right?
Interdasting.
But if not Yoda... whom?
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What? It's never implied that Luke went back to Yoda between ESB and ROTJ - in fact, it's made clear that he
didn't:
When Luke argued with Yoda and Obi Wan about leaving Degobah to help Han and Leia in ESB, he said, "Master Yoda, I promise to return and finish what I've begun. You have my word."
After finishing things up with Jabba in ROTJ, he hopped in his X-Wing and replied to R2's question with, "That's right, Artoo. We're going to the Dagobah system. I have a promise to keep... to an old friend."
The only thing we know for sure Luke did between ESB and ROTJ was build a new lightsaber.
When Luke returned to Yoda, Yoda said, "No more training do you require. Already know you that which you need."
and upon meeting Vader on Endor, Vader said, "I see you have constructed a new lightsaber. Your skills are complete."
In the EU, Luke went to Obi Wan's hut to meditate and construct the new lightsaber.