generalized-boolean—a generalized boolean.
The locally defined function next-method-p can be used
within the body forms (but not the lambda list)
defined by a method-defining form to determine
whether a next method exists.
The function next-method-p has lexical scope and indefinite extent.
Whether or not next-method-p is fbound in the
global environment is implementation-dependent;
however, the restrictions on redefinition and shadowing of
next-method-p are the same as for symbols in the COMMON-LISP package
which are fbound in the global environment.
The consequences of attempting to use next-method-p outside
of a method-defining form are undefined.