A selector is a Prolog term denoting a (path to a) subterm of a given term T. Let subterm(T,S) denote the subterm of T wrt. a selector S, and let N denote an integer. A selector then takes one of the following forms:

[…,N]Nth argument of the compound term subterm(T,[…])
[…,#N]Nth element of the list subterm(T,[…])

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