PUI - slide-puzzle neighbors

	(next t1 t2)
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    (next t2 t5)
    (next t3 t2)
    (next t3 t6)
    (next t4 t1)
    (next t4 t5)
    (next t4 t7)
    (next t5 t2)
    (next t5 t4)
    (next t5 t6)
    (next t5 t8)
    (next t6 t3)
    (next t6 t5)
    (next t6 t9)
    (next t7 t4)
    (next t7 t8)
    (next t8 t7)
    (next t8 t5)
    (next t8 t9)
    (next t9 t6)
    (next t9 t8)