;;; This handles the default rule.
 
 (define (maybe-default-ambiguous-tyvar type def module)
   (let ((classes (ntyvar-context type)))
    (and (not (null? classes)) ; this happens only during cleanup after an error
     (let ((non-standard? '#f)
 	  (numeric? '#f))
       (dolist (class classes)
 	(cond ((eq? (class-kind class) 'numeric)
 	       (setf numeric? '#t))
 	      ((not (eq? (class-kind class) 'standard))
 	       (setf non-standard? '#t))))
       (cond ((or non-standard? (not numeric?))
 	     (remember-context def
  	       (phase-error 'Non-defaultable-ambiguous-context
 "An ambiguous context, ~A, cannot be defaulted.~%Ambiguity in call to ~A~%"
                   classes def))
 	     '#f)
 	    (else
 	     (find-default-type type classes classes
 			(tuple-2-2 (assq module *default-decls*)))))))))
 
 (define (find-default-type tyvar classes all-classes defaults)
   (cond ((null? defaults)
 	 (phase-error 'no-default-applies
 	    "Ambiguous context: ~A~%No default applies.~%"
 	    all-classes)
 	 '#f)
 	((null? classes)
 	 (instantiate-tyvar tyvar (car defaults))
 	 '#t)
 	((type-in-class? (car defaults) (car classes))
 	 (find-default-type tyvar (cdr classes) all-classes defaults))
 	(else
 	 (find-default-type tyvar all-classes all-classes (cdr defaults)))))
 	 
 (define (type-in-class? ntype class)
   (let* ((ntype (expand-ntype-synonym ntype))
 	 (alg (ntycon-tycon ntype))
 	 (inst (lookup-instance alg class)))
     (if (eq? inst '#f)
 	'#f
 	(let ((res '#t))
 	  (do-contexts (c (instance-context inst)) (ty (ntycon-args ntype))
 	    (when (not (type-in-class? ty c))
 	      (setf res '#f)))
 	  res))))