diff --git a/goldfish/liii/base.scm b/goldfish/liii/base.scm
index 8daf379..ae77e33 100644
--- a/goldfish/liii/base.scm
+++ b/goldfish/liii/base.scm
@@ -15,7 +15,7 @@
 ;
 
 (define-library (liii base)
-(export == != display*)
+(export == != display* in?)
 (begin
 
 (define == equal?)
@@ -26,5 +26,18 @@
 (define (display* . params)
   (for-each display params))
 
+(define (in? elem l)
+  (cond ((list? l) (not (not (member elem l))))
+        ((vector? l)
+         (let loop ((i (- (vector-length l) 1)))
+           (if (< i 0)
+               #f
+               (if (== elem (vector-ref l i))
+                   #t
+                   (loop (- i 1))))))
+        ((and (char? elem) (string? l))
+         (in? elem (string->list l)))
+        (else (error 'type-error "type mismatch"))))
+
 ) ; end of begin
 )
diff --git a/tests/liii/base-test.scm b/tests/liii/base-test.scm
index 6e6984d..af9b605 100644
--- a/tests/liii/base-test.scm
+++ b/tests/liii/base-test.scm
@@ -17,6 +17,8 @@
 (import (liii check)
         (liii base))
 
+(check-set-mode! 'report-failed)
+
 (check (== (list 1 2) (list 1 2)) => #t)
 (check (!= (list 1 2) (list 1 2)) => #f)
 
@@ -26,4 +28,11 @@
       (display* "hello world" "\n")))
   => "hello world\n")
 
+(check (in? 1 (list )) => #f)
+(check (in? 1 (list 3 2 1)) => #t)
+(check (in? #\x "texmacs") => #t)
+(check (in? 1 (vector )) => #f)
+(check (in? 1 (vector 3 2 1)) => #t)
+(check-catch 'type-error (lambda () (in? 1 "123")))
+
 (check-report)