diff --git a/collects/mzlib/class.ss b/collects/mzlib/class.ss
index ac99417..526593c 100644
--- a/collects/mzlib/class.ss
+++ b/collects/mzlib/class.ss
@@ -2041,9 +2041,9 @@
     ;  corresponding method is overridden in the object's class relative to
     ;  the primitive class, #f otherwise.
     ;
-    ; When a primitive class have a primitive superclass, the
-    ;  struct:prim maker is responsible for ensuring that the returned
-    ;  struct items match the supertype predicate.
+    ; When a primitive class has a superclass, the struct:prim maker
+    ;  is responsible for ensuring that the returned struct items match
+    ;  the supertype predicate.
 
     (compose-class name
 		   (or super object%)
diff --git a/collects/mzlib/string.ss b/collects/mzlib/string.ss
index a83a7be..b3b1e29 100644
--- a/collects/mzlib/string.ss
+++ b/collects/mzlib/string.ss
@@ -101,13 +101,9 @@
   
   (define expr->string
     (lambda (v)
-      (let* ([s ""]
-	     [write-to-s
-	      (lambda (str)
-		(set! s (string-append s str)))]
-	     [port (make-output-port write-to-s (lambda () #f))])
+      (let ([port (open-output-string)])
 	(write v port)
-	s)))
+	(get-output-string port))))
   
   (define regexp-quote
     (opt-lambda (s [case-sens? #t])
diff --git a/collects/mzlib/thread.ss b/collects/mzlib/thread.ss
index 17e89d5..0bfdce1 100644
--- a/collects/mzlib/thread.ss
+++ b/collects/mzlib/thread.ss
@@ -97,17 +97,27 @@
 	    thunk
 	    (lambda () (semaphore-post sema))))))))
 
-  (define (copy-port src dest)
+  (define (copy-port src dest . dests)
+    (unless (input-port? src)
+      (raise-type-error 'copy-port "input-port" src))
+    (for-each
+     (lambda (dest)
+       (unless (output-port? dest)
+	 (raise-type-error 'copy-port "output-port" dest)))
+     (cons dest dests))
     (let ([s (make-string 4096)])
       (let loop ()
 	(let ([c (read-string-avail! s src)])
 	  (unless (eof-object? c)
-	    (let loop ([start 0])
-	      (unless (= start c)
-		(let ([c2 (write-string-avail s dest start c)])
-		  (loop (+ start c2)))))
+	    (for-each
+	     (lambda (dest)
+	       (let loop ([start 0])
+		 (unless (= start c)
+		   (let ([c2 (write-string-avail s dest start c)])
+		     (loop (+ start c2))))))
+	     (cons dest dests))
 	    (loop))))))
-
+  
   (define merge-input
     (case-lambda
      [(a b) (merge-input a b 4096)]