A character string which will be used as the prefix for the newly generated symbol.
This function generates a symbol which does not currently exist by attaching a unique number to the end of the prefix_string. If the prefix_string is nil, use a default prefix.
Gamma> gensym("tag");
tag1
Gamma> gensym();
tmp_sym2
Gamma> tag3 = 1;
1
Gamma> gensym("tag");
tag4
Gamma>