diff --git a/mes.c b/mes.c
index 119979bc..655abf0b 100644
--- a/mes.c
+++ b/mes.c
@@ -477,6 +477,12 @@ builtin_p (scm *x)
     ? &scm_t : &scm_f;
 }
 
+scm *
+boolean_p (scm *x)
+{
+  return (x == &scm_t || x == &scm_f) ? &scm_t : &scm_f;
+}
+
 scm *
 char_p (scm *x)
 {