Exercise 4.61.  The following rules implement a next-to relation that 
finds adjacent elements of a list:

(rule (?x next-to ?y in (?x ?y . ?u)))

(rule (?x next-to ?y in (?v . ?z))
      (?x next-to ?y in ?z))

What will the response be to the following queries?

(?x next-to ?y in (1 (2 3) 4))

(?x next-to 1 in (2 1 3 1))

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(1 next-to (2 3) in (1 (2 3) 4))

(2 next-to 1 in (2 1 3 1))

(Is there some hidden subtlety to this question that I'm missing?)