Controls the attempt to detect circularity and sharing in an object
the printing process merely proceeds by recursive descent without attempting
to detect circularity and sharing.
the printer will endeavor to detect cycles and sharing
in the structure to be printed,
and to use #n= and #n#
syntax to indicate the circularities or shared components.
If true, a user-defined
objects to the supplied stream using write, prin1,
princ, or format and expect circularities and sharing
to be detected and printed using the #n# syntax.
If a user-defined
prints to a stream other than the one
that was supplied, then circularity detection starts over for that stream.
Note that implementations should not use #n# notation
when the Lisp reader would automatically assure sharing without it
(e.g., as happens with interned symbols).