"\platform{UNIX}" should really be spelled "\platform{Unix}".
diff --git a/Doc/lib/libposix.tex b/Doc/lib/libposix.tex
index 0918ad0..c334893 100644
--- a/Doc/lib/libposix.tex
+++ b/Doc/lib/libposix.tex
@@ -2,7 +2,7 @@
          The most common \POSIX{} system calls}
 
 \declaremodule{builtin}{posix}
-  \platform{UNIX}
+  \platform{Unix}
 \modulesynopsis{The most common \POSIX{} system calls (normally used
 via module \module{os}).}