+;;;--------------------------------------------------------------------------
+;;; Metaclass inference.
+
+(export 'select-minimal-class-property)
+(defun select-minimal-class-property (supers key order default what
+ &key (present (lambda (x)
+ (format nil "`~A'" x)))
+ allow-empty)
+ "Return the minimal partially-ordered key from the SUPERS.
+
+ KEY is a function of one argument which returns some interesting property
+ of a class. The keys are assumed to be partially ordered by ORDER, a
+ function of two arguments which returns non-nil if its first argument
+ precedes its second. If there is a unique minimal key then return it;
+ otherwise report a useful error and pick some candidate in an arbitrary
+ way; the DEFAULT may be chosen if no better choices are available. If
+ ALLOW-EMPTY is non-nil, then no error is reported if there are no SUPERS,
+ and the DEFAULT choice is returned immediately.
+
+ In an error message, the keys are described as WHAT, which should be a
+ noun phrase; keys are filtered through PRESENT, a function of one
+ argument, before presentation.
+
+ The function returns two values: the chosen value, and a flag which is
+ non-nil if it was chosen without errors."
+
+ (let ((candidates (partial-order-minima (mapcar key supers) order)))
+ (cond ((and (null candidates) allow-empty)
+ (values default t))
+ ((and candidates (null (cdr candidates)))
+ (values (car candidates) t))
+ (t
+ (cerror* "No obvious choice for implicit ~A: ~
+ ~{~#[root classes must specify explicitly~:;~
+ candidates are ~
+ ~#[~;~A~;~A and ~A~:;~@{~A, ~#[~;and ~A~]~}~]~]~:}"
+ what (mapcar present candidates))
+ (dolist (candidate candidates)
+ (let ((super (find candidate supers :key key)))
+ (info-with-location super
+ "Direct superclass `~A' defined here ~
+ has ~A ~A"
+ super what (funcall present candidate))))
+ (values (if candidates (car candidates) default) nil)))))
+