+(define (find-entities-by-components entity-set clist)
+ (cond ((null? clist) '())
+ (else
+ (let ((e (hash-ref (cdr entity-set) (car clist) '()))
+ (e* (find-entities-by-components entity-set (cdr clist))))
+ (if (null? e*)
+ e
+ (lset-intersection eq? e e*))))))
+
+(define-syntax make-system
+ (syntax-rules ()
+ ((_ ((name (component-type ...)) ...) form ...)
+ (lambda (entity-set)
+ (let ((name (map (lambda (x)
+ (cons (car x)
+ (filter (lambda (x)
+ (memq (car x) '(component-type ...)))
+ (cdr x))))
+ (map (lambda (x)
+ (entity-ref x entity-set))
+ (find-entities-by-components entity-set '(component-type ...)))))
+ ...)
+ form
+ ...)))))
+
+(define-syntax define-system
+ (syntax-rules ()
+ ((_ system-name ((name (component-type ...)) ...) form ...)
+ (define system-name
+ (make-system ((name (component-type ...)) ...)
+ form
+ ...)))))
+
+(define (composed-systems-result results)
+ (let ((changes (filter (lambda (r) (entities-changes? r)) results)))
+ (cond ((null? changes)
+ (car results))
+ (else
+ (append-changes changes)))))