2 files changed, 12 insertions, 2 deletions
diff --git a/README.md b/README.md
index 7f3492f..decd7b5 100644
--- a/README.md
+++ b/README.md
@@ -187,11 +187,19 @@ otherwise `f`:
     (singul t)                ; t
     (singul '(a b))           ; f
 
-`(zerop x)` returns `t` if `x` is the number zero, otherwise `f`:
+`(zerop x)` returns `t` if `x` is the number positive zero, otherwise `f`:
 
     (zerop +0)                ; t
     (zerop +1)                ; f
-    (zerop t)                 ; error: t is not a number
+    (zerop -0)                ; f
+    (zerop 'a)                ; f
+
+`(plural x)` returns `t` if `x` is not the number positive zero, otherwise `f`:
+
+    (plural +0)               ; f
+    (plural +1)               ; t
+    (plural -0)               ; t
+    (plural 'a)               ; t
 
 ### Logical operations
 
diff --git a/liblali.c b/liblali.c
index a8d8adc..a816ad8 100644
--- a/liblali.c
+++ b/liblali.c
@@ -1672,6 +1672,8 @@ static char *stdlib = LISP(
 
   (defun zerop (x) (not (diff x +0)))
 
+  (defun plural (x) (diff x +0))
+
   (defmacro and args
     (fill ((algo args) t)
           ((algo (cdr args)) (car args))