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|library(gensym): Generate unique symbols
Gensym (Generate Symbols) 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
catch22 is not an already defined atom and therefore one
must be aware these atoms are only unique in an isolated context.
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.