Hacking gram.y Error syntax error at or near "MERGEJOIN"

    I'm doing a hacking project which hacks the gram.y file and some other so that postgres can execute some query plan language to run a particular plan. I did some modifications and try run it. It compiled and installed successfully. But when I was trying to run a MergeJoin operation with keyword MERGEJOIN, it says "ERROR: syntax error at or near 'MERGEJOIN'". It seems postgres didn't recognize this keyword. Is it caused by any error inside gram.y? Or how can I trace this error?

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Best Regards
Huang Qi Victor