a non-negative integer.
implementation-dependent.
A number which will be used in constructing the name of
the next symbol generated by the function gensym.
*gensym-counter* can be either assigned or bound
at any time, but its value must always be a non-negative integer.
gensym.
The ability to pass a numeric argument to gensym has been deprecated;
explicitly binding *gensym-counter* is now stylistically preferred.