CLHS: Section 7.6.6.1.3

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## 7.6.6.1.3 Applying method combination to the sorted list of applicable methods

In the simple case---if standard method combination is used and all applicable methods are primary methods---the effective method is the most specific method. That method can call the next most specific method by using the function call-next-method. The method that call-next-method will call is referred to as the next method. The predicate next-method-p tests whether a next method exists. If call-next-method is called and there is no next most specific method, the generic function no-next-method is invoked.

In general, the effective method is some combination of the applicable methods. It is described by a form that contains calls to some or all of the applicable methods, returns the value or values that will be returned as the value or values of the generic function, and optionally makes some of the methods accessible by means of call-next-method.

The role of each method in the effective method is determined by its qualifiers and the specificity of the method. A qualifier serves to mark a method, and the meaning of a qualifier is determined by the way that these marks are used by this step of the procedure. If an applicable method has an unrecognized qualifier, this step signals an error and does not include that method in the effective method.

When standard method combination is used together with qualified methods, the effective method is produced as described in Section 7.6.6.2 (Standard Method Combination).

Another type of method combination can be specified by using the `:method-combination` option of defgeneric or of any of the other operators that specify generic function options. In this way this step of the procedure can be customized.

New types of method combination can be defined by using the define-method-combination macro.

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