Installs an entry into the pprint dispatch table which is table.
is the key
of the entry. The first action of set-pprint-dispatch is to remove any
pre-existing entry associated with type-specifier. This guarantees that
there will never be two entries associated with the same type specifier
in a given pprint dispatch table. Equality of type specifiers is
tested by equal.
Two values are associated with each type specifier in a
pprint dispatch table: a function and a priority.
The function must accept two arguments: the stream to which output
is sent and the object to be printed. The function should
pretty print the object to the stream. The function
can assume that object satisfies the type given by type-specifier.
The function must obey *print-readably*.
Any values returned by the function are ignored.
Priority is a priority to resolve conflicts
when an object matches more than one entry.
It is permissible for function to be nil. In this situation,
there will be no type-specifier entry in table after