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Common Lisp the Language, 2nd Edition
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The defclass form for a class provides a total ordering on
that class and its direct superclasses. This ordering is called the
local precedence order. It is an ordered list of the class and
its direct superclasses. The class precedence list for a class
C is a total ordering on C and its superclasses that
is consistent with the local precedence orders for C and its
superclasses.
A class precedes its direct superclasses, and a direct superclass
precedes all other direct superclasses specified to its right in the
superclasses list of the defclass form. For every class
C, define
where 
are the direct
superclasses of C in the order in which they are mentioned in
the defclass form. These ordered pairs generate the total
ordering on the class C and its direct superclasses.
Let 
be the set of
C and its superclasses. Let R be
The set R may or may not generate a partial ordering,
depending on whether the 
,
, are consistent; it is
assumed that they are consistent and that R generates a partial
ordering. When the are not
consistent, it is said that R is inconsistent.
To compute the class precedence list for C, topologically
sort the elements of 
with
respect to the partial ordering generated by R. When the
topological sort must select a class from a set of two or more classes,
none of which are preceded by other classes with respect to R,
the class selected is chosen deterministically, as described below. If
R is inconsistent, an error is signaled.
Next: Topological Sorting
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