Package prefixes are printed if necessary.
The rules for package prefixes are as follows.
When the symbol is printed, if it is in the KEYWORD package,
then it is printed with a preceding colon; otherwise, if
it is accessible in the current package, it is printed without any
package prefix; otherwise, it is printed with a package prefix.
A symbol that is apparently uninterned is printed
preceded by "#:"
if *print-gensym* is true and printer escaping is enabled; if *print-gensym* is false or printer escaping is disabled,
then the symbol is printed without a prefix, as if it were in the current package.
Because the #: syntax does not intern the
following symbol, it is necessary to use circular-list syntax
if *print-circle* is true and
the same uninterned symbol appears several times in an expression
to be printed. For example, the result of
(let ((x (make-symbol "FOO"))) (list x x))
would be printed as (#:foo #:foo) if *print-circle*
were false, but as (#1=#:foo #1#) if *print-circle*
were true.
A summary of the preceding package prefix rules follows:
foo:barfoo:bar is printed when symbol bar
is external in its home package foo
and is not accessible in the current package.
foo::barfoo::bar is printed when bar is internal in its home package
foo and is not accessible in the current package.
:bar:bar is printed when the home package of bar is the KEYWORD package.
#:bar is printed when bar is apparently uninterned,
even in the pathological case that bar
has no home package but is nevertheless somehow accessible
in the current package.