1/*  Part of SWI-Prolog
    2
    3    Author:        Jan Wielemaker
    4    E-mail:        J.Wielemaker@vu.nl
    5    WWW:           http://www.swi-prolog.org
    6    Copyright (c)  2019, VU University Amsterdam
    7    All rights reserved.
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    9    Redistribution and use in source and binary forms, with or without
   10    modification, are permitted provided that the following conditions
   11    are met:
   12
   13    1. Redistributions of source code must retain the above copyright
   14       notice, this list of conditions and the following disclaimer.
   15
   16    2. Redistributions in binary form must reproduce the above copyright
   17       notice, this list of conditions and the following disclaimer in
   18       the documentation and/or other materials provided with the
   19       distribution.
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   21    THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
   22    "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
   23    LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
   24    FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
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   33*/
   34
   35:- module(setof,
   36          [ excess_vars/4	% +Term, +ExistVarTerm, +AddVarList, -VarList
   37          ]).   38:- use_module(library(lists)).

 excess_vars(+Term, +ExistVarTerm, +AddVarList, -VarList)

Returns in VarList the list of (free) variables found in Term concatenated to the end of AddVarList. (In normal usage AddVarList is passed in as an empty list.) ExistVarTerm is a term containing variables assumed to be quantified in Term so none of these variables are returned in the resulting list (unless they are in AddVarList.) Subterms of Term of the form (VarTerm^SubTerm) are treated specially: all variables in VarTerm are assumed to be quantified in SubTerm, and so no occurrence of these variables in SubTerm is collected into the resulting list.

   52excess_vars(Term, ExistVarTerm, AddVarList, VarList) :-
   53    '$free_variable_set'(ExistVarTerm^Term, _Goal, Vars0),
   54    append(AddVarList, Vars0, VarList)