and $y$ are considered equal if and only if @|(funcall @<test> (funcall
@<key> $x$) (funcall @<key> $y$))| returns non-nil.
+ The @<report> function is called as @|(funcall @<report> @<duplicate>
+ @<previous>)|. Duplicates are reported in order; the @<previous> item is
+ always the first matching item in the sequence.
+
This function will work for arbitrary @<test> functions, but it will run
- much more efficiently if @<test> is @|eq|, @|eql|, @|equal|, or @|equalp|
- (because it can use hash-tables).
+ much more efficiently if @<test> is @|eq|, @|eql|, @|equal|, or @|equalp|,
+ because it can use hash-tables. (The generic implementation for lists is
+ especially inefficient.)
\end{describe}