+The Name may only consist of alphanumeric and underscore characters.
+
+If
+.Va Name
+starts with a
+.Li $ ,
+then the contents of the environment variable that follows will be used.
+In that case, invalid characters will be converted to underscores.
+If
+.Va Name
+is
+.Li $HOST ,
+but no such environment variable exist, the hostname will be read using the gethostnname() system call.