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:

S | subterm(T,S) |

`[]` | T |

`[…,` | Nth argument of the compound term subterm(T,`[…]` ) |

`[…,#` | Nth element of the list subterm(T,`[…]` ) |

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