function-name—a function name.
The keyword arguments correspond to the option arguments of
defgeneric, except that the :method-class and
:generic-function-class arguments can be class objects
as well as names.
Method-combination—method combination object.
Environment—the same as the &environment argument
to macro expansion functions and is used to distinguish between compile-time
and run-time environments.
generic-function—a generic function object.
The function ensure-generic-function is used to define
a globally named generic function with no methods
or to specify or modify options and declarations that pertain to
a globally named generic function as a whole.
If function-name is not fbound in the global environment,
a new
generic function is created.
If
(fdefinition function-name)
is an ordinary function,
a macro,
or a special operator,
an error is signaled.
If function-name
is a list, it must be of the
form (setf symbol).
If function-name specifies a generic function that has a
different value for any of the following arguments,
the generic function is modified to have the new value:
:argument-precedence-order, :declare, :documentation,
:method-combination.
If function-name specifies a generic function that has a different value for the :lambda-list argument, and the new value is congruent with the lambda lists of all existing methods or there are no methods, the value is changed; otherwise an error is signaled.
If function-name specifies a generic function that has a
different value for the :generic-function-class argument and if
the new generic function class is compatible with the old,
change-class is called to change the class of the
generic function;
otherwise an error is signaled.
If function-name specifies a generic function that has a different value for the :method-class argument, the value is changed, but any existing methods are not changed.
Existing function binding of function-name.
If
(fdefinition function-name)
is an ordinary function, a macro, or a special operator,
an error of type error is signaled.
If function-name specifies a
generic function that has a
different value for the :lambda-list argument, and the new value
is not congruent with the lambda list of any existing
method,
an error of type error is signaled.
If function-name specifies a
generic function that has a
different value for the :generic-function-class argument and if
the new generic function class not is compatible with the old,
an error of type error is signaled.