I solved it by
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×forcing the serpent into the notch by trapping it in its own body.
However, it might be easier to
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×Play around with the serpent until it is a bit shorter and then manipulate it in the usual way into the notch by
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×having it follow you closely from the north along the east column. When you are on the same row as the notch, step west and then north. If the serpent has the right movement preference at the time, then it will go into the notch. If not, then try again. Eventually you'll get it.
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