Exercise solution:
Generation of functions with a very flexible signature

 
(define (grouping-abstraction-by-predicates collector pred1 pred2)
  (lambda parameters
     (let* ((res-3 (group-in-3 parameters pred1 pred2 '() '() '()))
            (list-of-pred-1-elements (car res-3))
            (list-of-pred-2-elements (cadr res-3))
            (other-elements (caddr res-3)))
       (apply collector (list list-of-pred-1-elements list-of-pred-2-elements other-elements)))))

(define (group-in-3 parameters pred1 pred2 lst1 lst2 lst3)
  (cond ((null? parameters) (list (reverse lst1) (reverse lst2) (reverse lst3)))
        ((pred1 (car parameters)) (group-in-3 (cdr parameters) pred1 pred2 (cons (car parameters) lst1) lst2 lst3))
        ((pred2 (car parameters)) (group-in-3 (cdr parameters) pred1 pred2 lst1 (cons (car parameters) lst2) lst3))
        (else (group-in-3 (cdr parameters) pred1 pred2 lst1 lst2 (cons (car parameters) lst3)))))

(define  (sum number-list)
  (cond ((null? number-list) 0)
        (else (+ (car number-list) (sum (cdr number-list))))))

(define h (grouping-abstraction-by-predicates (lambda (l1 l2 l3) (list (sum l1) (apply string l2))) number? char?))