/usr/share/doc/swi-prolog-doc/Manual/gensym.html

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<H2><A NAME="sec:A.10"><SPAN class="sec-nr">A.10</SPAN> <SPAN class="sec-title">library( 
gensym ): Generate unique identifiers</SPAN></A></H2>

<A NAME="gensym"></A>

<P>Gensym (<B>Gen</B>erate <B>Sym</B>bols) is an old library for 
generating unique symbols (atoms). Such symbols are generated from a 
base atom which gets a sequence number appended. Of course there is no 
guarantee that `catch22' is not an already defined atom and therefore 
one must be aware these atoms are only unique in an isolated context.

<P>The SWI-Prolog gensym library is thread-safe. The sequence numbers 
are global over all threads and therefore generated atoms are unique 
over all threads.

<DL>
<DT class="pubdef"><A NAME="gensym/2"><STRONG>gensym</STRONG>(<VAR>+Base, 
-Unique</VAR>)</A></DT>
<DD class="defbody">
Generate a unique atom from base <VAR>Base</VAR> and unify it with <VAR>Unique</VAR>.
<VAR>Base</VAR> should be an atom. The first call will return &lt;<VAR>base</VAR>&gt;1 
, the next &lt;<VAR>base</VAR>&gt;2 , etc. Note that this is no warrant 
that the atom is unique in the system.</DD>
<DT class="pubdef"><A NAME="reset_gensym/1"><STRONG>reset_gensym</STRONG>(<VAR>+Base</VAR>)</A></DT>
<DD class="defbody">
Restart generation of identifiers from <VAR>Base</VAR> at &lt;<VAR>Base</VAR>&gt;1. 
Used to make sure a program produces the same results on subsequent 
runs. Use with care.</DD>
<DT class="pubdef"><A NAME="reset_gensym/0"><STRONG>reset_gensym</STRONG></A></DT>
<DD class="defbody">
Reset gensym for all registered keys. This predicate is available for 
compatibility only. New code is strongly advice to avoid the use of 
reset_gensym or at least to reset only the keys used by your program to 
avoid unexpected site-effects on other components.
</DD>
</DL>

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