a pprint dispatch table.
implementation-dependent, but the initial entries all use a special class of priorities that have the property that they are less than every priority that can be specified using set-pprint-dispatch, so that the initial contents of any entry can be overridden.
The pprint dispatch table which currently controls the pretty printer.
*print-pretty*, Section 18.104.22.168 (Pretty Print Dispatch Tables)
The intent is that the initial value of this variable should cause `traditional' pretty printing of code. In general, however, you can put a value in *print-pprint-dispatch* that makes pretty-printed output look exactly like non-pretty-printed output. Setting *print-pretty* to true just causes the functions contained in the current pprint dispatch table to have priority over normal print-object methods; it has no magic way of enforcing that those functions actually produce pretty output. For details, see Section 22.214.171.124 (Pretty Print Dispatch Tables).